Abstract: Vapor phase contact reaction between water and benzal chloride or a substitute expressed by C.sub.6 H.sub.(5-n) X.sub.n CHCl.sub.2, wherein X representing a halogen atom or a trifluoromethyl group and n being 1 or 2, to form benzaldehyde or a substitute expressed by C.sub.6 H.sub.(5-n) X.sub.n CHO can efficiently be achieved by using activated carbon treated with an acid such as sulfuric acid or impregnated with a metal chloride such as ferric chloride and/or a metal sulfate such as cupric sulfate as catalyst. The activated carbon catalyst long retains its high activity even when the starting material has trifluoromethyl group, which is liable to undergo partial decomposition with formation of hydrogen fluoride.