A thin-layered V-P oxide (VPO) catalyst deposited, 0.64 g/800 cm(2) of a 918-m(2)/m(3) 95.6% empty structured carrier (20-leaflet rosettes attached to rounded 24.4/12.0/2.2 mm rings) gave 0.7 g maleic anhydride/(g(.)h), i.e., more than any granular VPO so far, in the oxidation of 1 L(S.T.P)/h n-butane at 1 atm. and 360-400degreesC with 50-100 L air/h. Examination of 17 structural carriers showed the heat-flow efficiency to be controlled by carrier's streamlined shape and an axial vortex formed by the flowing reactant gas.