Frémys work included investigations of osmic acid, the ferrates, stannates, plumbates, and other oxometallates, as well as ozone; attempts to obtain free fluorine by the electrolysis of fused fluorides; and the discovery of anhydrous hydrofluoric acid and a series of acides sulphazotés, the precise nature of which long remained a matter of discussion. He also studied the coloring of leaves and flowers, the composition of bone, cerebral matter, and other animal substances, and the processes of fermentation, in which he was an opponent of Pasteurs views.
Keenly alive to the importance of the technical applications of chemistry, Frémy devoted special attention as a teacher to the training of industrial chemists. In this field he contributed to our knowledge of the manufacture of iron and steel, sulfuric acid, glass, and paper, and in particular worked at the saponification of fats with sulfuric acid and the utilization of palmitic acid for candle-making. In the later years of his life he applied himself to the problem of obtaining alumina in the I crystalline form, and succeeded in making rubies identical with the natural gem not merely in chemical composition but also in physical properties.