A 2-(substituted aryl)propionaldehyde of the formula, ##STR1## (wherein Ar denotes ##STR2## is produced in high yield by reacting a methyl (substituted aryl) ketone of the formula ##STR3## with phenyldimethylsulfonium methylsulfate of the formula, ##STR4## in the presence of alkali metal hydroxide to obtain a reaction mixture containing 2-(substituted aryl)1,2-epoxypropane of the formula, ##STR5## and thioanisole, and subjecting the reaction mixture to contact with anhydrous MgCl.sub.2. The propionaldehyde compound is also produced in high yield by contacting the epoxypropane compound with anhydrous MgCl.sub.2 in the presence of a soft nucleophile such as a sulfide, a thiol and a phosphine.