At its disappearance in 1968, Seine-et-Oise consisted of 688 suburban and rural communes completely surrounding the Seine département although it was at its narrowest just east of Seine between that département and the Seine-et-Marne département which still exists today. It had an area of 5,658 km² (2,184 sq. miles). The division of Seine-et-Oise into arrondissements changed many times. At its disappearance it had ten arrondissements: Argenteuil, Étampes, Mantes, Montmorency, Palaiseau, Pontoise, Le Raincy, Rambouillet, Saint-Germain-en-Laye, and Versailles.
In detail, the splitting up of the Seine-et-Oise département was carried out like this: 262 communes in the central part of the département became the Yvelines département, with Versailles as the préfecture. The official number 78 which was used for Seine-et-Oise was given to the new Yvelines département, which is the largest chunk of the former Seine-et-Oise (40% of the area of Seine-et-Oise). 198 communes in the south of Seine-et-Oise (32% of the area of Seine-et-Oise) became the Essonne département, and the official number 91 was assigned to this département (a number previously used for the Alger département in French Algeria). 185 communes in the north of Seine-et-Oise (22% of the area of Seine-et-Oise) became the Val-d'Oise département, and the official number 95 was assigned to this département (a number never used before).
Of the remaining 6% of Seine-et-Oise, 18 communes were grouped with 29 communes of the Seine département to create the Val-de-Marne département. 16 communes of Seine-et-Oise were grouped with 24 communes of the Seine département to create the Seine-Saint-Denis département. Finally, the last 9 communes of Seine-et-Oise were grouped with 27 communes of the Seine département to create the Hauts-de-Seine département.
Thus, it should be noted that Yvelines, Val-d'Oise, and Essonne are altogether smaller than the former Seine-et-Oise département (5,658 km² for the Seine-et-Oise département vs. 5,334 km² for the three départements).