A substituted benzaldehyde is obtained by the catalytic vapor-phase oxidation of methoxy, tert-butyl-, or phenoxy- ! substituted toluene with a molecular oxygen containing gas in the presence of a catalyst composition of the formula V a X b Y c O d , wherein X is at least one of Na, K, Rb, Cs, and T/ (except the case where K is used alone), Y is at least one of Mg, Ca, Sr, Ba, Nb, Ta, P, Sb, Bi, Cu, Ag, Ti, Fe, Co, Ni, Sn, Ce and Zn, and a, b, c and d are atomic proportions of respective elements.