An n.m.r. technique was used to measureKH/KDof ketal formation in methylene chloride solution for acetone and phenylacetone. For acetone,KH/KD = 0.95 per D for methyl ketal formation. Phenylacetone-1,1-d2, -3,3,3-d3, and -1,1,1,3,3-d5were examined for both dimethyl and ethylene ketal formation. For both ketals,KH/KD = 0.91 per D, for each ketone. Liquid scintillation counting was used to examineKH/KTfor ketal formation in methylene chloride. An extraordinarily large isotope effect was found for ethylene ketal formation in 1,3-diphenyl-acetone. HereKH/KT = 0.67 for one tritium atom. This isotope effect is too large to arise from hyperconjugative effects alone. It must have a substantial steric component.