Vanadyl hydrogenphosphate sesquihydrate (VOHPO4 . 1.5H(2)O) was prepared by reduction of VOPO4 . 2H(2)O suspended in refluxed l-butanol and activated at 480 degrees C over 10 h on-stream to give (VO)(2)P2O7, which revealed high specific activity per unit surface area in the vapor-phase oxidation of n-butane. Cobalt-modified (VO)(2)P2O7 catalysts were prepared in the present work by intercalating sesquihydrate precursor with cobaltous acetate employing l-butanol as solvent. The catalysts prepared from cobalt-intercalated precursor showed high specific activity, but both selectivities to maleic anhydride and surface area decreased with increasing cobalt-contents. Presumably, intercalated cobalt species retarded topotactic dehydration of layered precursor to afford crystalline active species (VO)(2)P2O7, leading to an increase in unfavorable combustion activity due to pentavalent beta-VOPO4 species. The activity of catalysts prepared for reference by kneading vanadyl hydrogenphosphate hydrates with an additive Co2P2O7 was interpreted as the variation in surface area, and cobalt has none of positive effects in these catalysts.