Collet-Descotils was educated at the Collège du Bois of the University of Caen. About 1789 he was taken by his father to Paris, where he studied physics under J. A. C. Charles and chemistry under Vauquelin.
In 1792 Collet-Descotils was drafted into the navy and served at Cherbourg; he obtained his discharge in 1794 after passing the entrance examination for the Ècole des Mines in Paris. The Société Philomathique accepted Collet-Descotils as a member in 1796, and he was invited to join the scientific group, led by Berthollet and Monge, that accompanied Napoleon’s expedition to Egypt in 1798. He stayed in Egypt until 1801 as a member of the Institut d’Égypte.
On returning to France, Collet-Descotils succeeded Vauquelin as a professor at the École des Mines; he gave only one lecture course, however, for teaching ceased there when two mining schools were founded in the provinces. Nevertheless, he remained as director of the laboratory; and in 1809 he became one of the engineers in chief in the Department of Mines. He was appointed an acting director of the École des Mines when it reopened (on a limited basis) in Paris in 1814, only a year before his death.
Collet-Descotils joined the editorial board of Annales de Chimie in 1804 and later became its secretary; and from 1807 he was a member of the Société d’Arcueil, which met at Berthollet’s country house. He was respected as a very competent chemist; but although he was in touch with leading French scientists, he did no outstanding research.
In the course of his official duties, Collet-Descotils visited mines in various parts of France and Italy, investigated metallurgical problems, and analyzed numerous ores. In 1805 he examined a brown lead ore from Mexico that A. M. del Rio believed to contain a new metal. He concluded that it was chromium, and other chemists agreed; thus the true identity of vanadium remained unknown until its rediscovery in 1830 by N. G. Sefström.
Collet-Descotils was more fortunate in his research on crude platinum, which contained a black constituent that normally seemed insoluble in aqua regia. In 1803 he showed that it dissolved slowly, especially in the presence of excess nitric acid, to form a solution that gave a red precipitate with ammonium chloride. Pure platinum gave a yellow precipitate, so a new metal must be present in the red compound. Fourcroy and Vauquelin independently reached the same conclusion, and a more detailed study by Smithson Tennant revealed that there were in fact two metals in the less soluble part of crude platinum-iridium (which gave the red compound) and osmium. In 1807 Collet-Descotils found that iridium could be precipitated from a platinum solution by making it alkaline and oxidizing it by exposure to air; it was only in the twentieth century, however, that this procedure, in an improved form, was generally adopted by platinum refiners.
Collet-Descotils left a widow and two sons.