Potassium ferrocyanide, also known as yellow prussiate of potash or potassium hexacyanoferrate(II), is a coordination compound of formula K4[Fe(CN)6]·3H2O, which forms lemon-yellow monoclinic crystals at room temperature, and which decomposes at its boiling point. It is insoluble in alcohol but a litre of water can dissolve just under three hundred grams of the crystals, and the solution can react with acid to release hydrogen cyanide (HCN) gas. The resulting HCN gas boils at 26 °C and, being lighter than air (with a gaseous density of 0.94), quickly evaporates clear of the release point. Potassium ferrocyanide itself is only slightly toxic.
On February 20, 2002 four Moroccans were arrested while in possession of detailed maps of the United States embassy in Rome and the Rome water supply network, as well as four kilograms of potassium ferrocyanide.