The condensation of acetone with glycerol has been carried out in a neutral medium, at a low temperature, and the product shown to consist entirely of the five-membered isopropylidene-glycerol ketal. Based on this fact, the use of the substance for the determination of the structure of fats is therefore warranted. Isopropylidene-glycerol and its acetate readily hydrolyse on heating with water.No satisfactory explanation can, as yet, be offered for the non-formation of the six-membered ketal under the conditions outlined.Acetone, as previously shown by other workers, is capable of forming a six-membered cyclic ketal with glycerol-β-methyl ether, dihydroxy acetone, pentaerythritol and certain carbohydrates.