French chemist best known for his investigations of fluorine compounds. In 1831 he entered the laboratory of Joseph-Louis Gay-Lussac and, after holding several teaching posts, succeeded Gay-Lussac in the chemistry chair at the Museum of Natural History, Paris (1850), of which he became director (1879–91).

Frémy investigated osmic acid, ozone, and compounds of iron, tin, and lead, among others. He sought to isolate free fluorine and discovered hydrogen fluoride and a series of its salts. He studied the colouring matters of leaves and flowers and the composition of animal substances. He contributed to the technology of iron, steel, sulfuric acid, glass, paper, and, in particular, the saponification of fats. He sought to obtain crystals of aluminum oxide and succeeded in making rubies.