Abstract: In the process for the preparation of 1,4-naphthoquinone by vapor phase oxidation of naphthalene with a molecular oxygen containing gas using a solid catalyst consisting essentially of an inert carrier supporting thereon a catalyst composition comprising as essential active components vanadium pentoxide, an alkali metal sulfate and an alkali metal pyrosulfate, the improvement which comprises supplying to the raw material feed sulfur or a sulfur compound capable of yielding sulfur trioxide in a system as to maintain the equilibrium between the alkali metal sulfate and the alkali metal pyrosulfate in the catalyst composition and keep the proportion of the active components constant.