Unsaturated hydrocarbons having 4 to 6 carbon atoms are catalytically oxidized into maleic anhydride by using a catalyst, which consists essentially of oxides of (A) vanadium, (B) phosphorus, (C) titanium and (D) optionally at least one element selected from sodium, calcium, magnesium, iron, zirconium, boron, manganese, silver and molybdenum in the atomic ratios defined by the following formulaVP.sub.a Ti.sub.b X.sub.c O.sub.dwherein X is the element (D) set forth above and a=1.0 to 5.0, b=2.0 to 12.0 and c=0 to 1.0, and which catalyst is prepared by calcining a mixture of a vanadium-containing compound and titanium dioxide at a temperature of 650.degree. C. to 1,500.degree. C., incorporating into the mixture a phosphorus-containing compound and optionally the element "X"-containing compound and, then, heating the resulting mixture.