A method for the production of 3-cyano-3,5,5-trimethyl cyclohexanone by the reaction of isophorone with hydrogen cyanide in the presence of a quaternary ammonium salt or quaternary phosphonium salt and a basic compound as a catalyst in the form dissolved or dispersed in water or in the absence of water, which method comprises effecting said reaction by using said quaternary ammonium salt or quaternary phosphonium salt in an amount in the range of from 0.001 to 0.1 mol per mol of isophorone and said basic compound and in an amount in the range of from 0.5 to 3.0 mols per mol of said quaternary ammonium salt or quaternary phosphonium salt and, when water is used, using water in an amount in the range of 0.1 to 25% by weight based on the amount of isophorone and introducing hydrogen cyanidehydrocyanic acid at a ratio in the range of from 0.6 to 1.0 mol per mol of isophorone at a temperature in the range of from 90.degree. to 140.degree. C.