His early interest in botanical studies naturally led him to undertake journeys among the Alps, and from 1773 onwards he directed his attention to the geology and physics of that region. This work did much to clear up the topography of the snowy portions of the Alps, and to attract the attention of tourists to spots like Chamonix and Zermatt.
In 1760 he first visited Chamonix, and offered a reward to the first man to reach the summit of Mont Blanc (then unscaled). He made an unsuccessful attempt himself in 1785, by the Aiguille du Goûter route. Two Chamonix men, Dr Michel Paccard and Jacques Balmat attained the summit in 1786, by way of the Grands Mulets, and in 1787 Saussure himself had the delight of gaining the summit (the third ascent).
In 1788 he spent 17 days making observations on the crest of the Col du Géant (3,371 m). In 1774 he mounted the Crammont, and again in 1778, in which year he also explored the Valsorey glacier, near the Great St Bernard in 1776 he had ascended the Buet (3,096m).
In 1789 he visited the Pizzo Bianco (near Macugnaga) and crossed the Theodulpass (3,322 m) to Zermatt, which he was the first traveller to visit. On that occasion he climbed from the pass up the Klein Matterhorn (3,883m), while in 1792 he spent three days on the same pass, (not descending to Zermatt), making observations, and then visited the Theodulhorn (3,472m).
In 1780 he climbed the Roche Michel, above the Mont Cenis Pass. The descriptions of seven of his Alpine journeys, with his scientific observations gathered en route, were published by him in four quarto volumes, under the general title of Voyages dans les Alpes (1779-1796; there was an octavo issue in eight volumes, issued 1780-1796, while the non-scientific portions of the work were first published in 1834, and often since, under the title of Partie pittoresque des ouvrages de M. de Saussure).
The Alps formed the centre of Saussure's investigations. He saw them as the grand key to the true theory of the earth, and they gave him the opportunity for studying geology in a manner never previously attempted. The inclination of the strata, the nature of the rocks, the fossils and the minerals received close attention.
He acquired a thorough knowledge of the chemistry of the day; and he applied it to the study of minerals, water and air. Saussure's geological observations made him a firm believer in the Neptunian theory: he regarded all rocks and minerals as deposited from aqueous solution or suspension, and attached much importance to the study of meteorological conditions.
He carried barometers and boiling-point thermometers to the summits of the highest mountains, and estimated the relative humidity of the atmosphere at different heights, its temperature, the strength of solar radiation, the composition of air and its transparency. Then, following the precipitated moisture, he investigated the temperature of the earth at all depths to which he could drive his thermometer staves, the course, conditions and temperature of streams, rivers, glaciers and lakes, even of the sea.
In the Essai sur l'hygrométrie, published in 1783, he records experiments made with various forms of hygrometer in all climates and at all temperatures, and supports the claims of his hair hygrometer against all others. He invented and improved many kinds of apparatus, including the magnetometer, the cyanometer for estimating the blueness of the sky, the diaphanometer for judging of the clearness of the atmosphere, the anemometer and the mountain eudiometer.
His modifications of the thermometer adapted that instrument to many purposes: for ascertaining the temperature of the air he used one with a fine bulb hung in the shade or whirled by a string, the latter form being converted into an evaporometer by inserting its bulb into a piece of wet sponge and making it revolve in a circle of known radius, at a known rate; for experiments on the earth and in deep water he employed large thermometers wrapped in non-conducting coatings so as to render them extremely sluggish, and capable of long retaining the temperature once they had attained it.
With these instruments he showed that the bottom water of deep lakes is uniformly cold at all seasons, and that the annual heat wave takes six months to penetrate to a depth of 30 ft. in the earth. He recognized the immense advantages to meteorology of high-level observing stations, and whenever it was practicable he arranged for simultaneous observations being made at different altitudes for as long periods as possible.
It is perhaps as a geologist (it is said that he was the first to use the term "geology" - see the "Discours préliminaire" to vol i. of his Voyages, publ. in 1779) that Saussure worked most; and although his ideas on matters of theory were in many cases very erroneous he was instrumental in greatly advancing that science.
He constructed the first known Western solar oven in 1767.
De Saussure died in Geneva in 1799.
The genus of high alpine plants Saussurea, some adapted to growing in some of the most extreme high alpine climates tolerated by any plant, is named after him. The standard botanical author abbreviation Sauss. is applied to species he described.
His work as a mineralogist was also recognized. The Saussurite is named after him.
De Saussure was honoured by being depicted on the 20 Swiss franc banknote of the sixth issue of Swiss National Bank notes (1979-1995 when replaced by the eighth issue, and the notes were recalled in 2000) (These notes will become valueless on 1 May 2020).
His son Nicolas-Théodore de Saussure was a noted specialist in organic chemistry.
His daughter Albertine Necker de Saussure was a pioneer in the education of women.