Ketones are prepared with a high yield and a prolonged catalyst life, by reacting an aldehyde selected from the group consisting of straight chain or branched chain aliphatic aldehydes having a carbon number of 4-10 (inclusive) and aromatic aldehydes represented by the general formula of ##SPC1##Wherein n is 0-5 with a monocarboxylic acid selected from the group consisting of acetic acid and propionic acid, at a temperature of 200.degree.-600.degree.C, in the presence of a catalyst of zirconium oxide alone or a mixture of zirconium oxide and at least one kind of the metal oxide selected from the group consisting of alkali metal oxides and alkaline earth metal oxides.