Unpromoted and 1.2, 2.3 and 4.3 molar % Co:V cobalt-promoted vanadium-phosphorous-oxide (VPO) catalysts were synthesized via an organic route. The catalyst precursors were calcined and then conditioned in a reactor, forming the active vanadyl pyrophosphate (VO)(2)P2O7 phase, which was confirmed via X-ray diffraction studies (XRD). The effect of co-promotion on the yield of maleic anhydride (MA) from n-butane oxidation was examined at different temperatures and gas hourly space velocities (GHSV). 2.3% Co:V was the optimum promoter loading for a high yield towards MA. Higher GHSV's proved to enhance the selectivity towards MA. The catalysts were tested over a 200 It period, generally taking some 24 h to reach steady-state performance. The best performing catalyst yielded 45% MA at 275 degreesC and a GHSV of 7200 ml g(-1) h(-1), with an n-butane conversion of 73%, whilst all previously reported VPO catalysts produced far lower MA yields at this temperature. (C) 2004 Elsevier B.V. All rights reserved.