n-Butane, 1 L(S.T.P.)/h, was oxidized with 30-60 L/h air at 370-380degreesC in a 165-mm tube packed with a structured carrier-supported catalyst to yield maleic anhydride at a max. conversion of 45% and max. selectivity of 31%. Of the 9 metal structures studied, the best was rounded 24.4/12 x 2.2 mm rings, each carrying 20 rosettes (10-mm long leaflets 0.05 mm thin). The 20% Cr-5% Al steel rings were annealed 110 h at 1000-1050degreesC (to have 0.5-1-mum gamma-Al2O3 crystallites formed on the surface) and coated with 2 mg/cm(2) of a TiO2+Al2O3 washcoat. The catalyst was a conventional V-P oxide type prepared on the carrier surface coated with a 0.8 mg/cm(2) of a suspension of precursors (NH4O3 + H3PO4). In the catalyst, thick surface layers were rich in P and, V, P:V = 1:similar to1; other layers had 1:1.5-1:8.