The partial oxidation of 1,3-butadiene has been investigated over VMoO catalysts synthesized by sol-gel techniques. Surface areas were 9-14 m(2)/g, and compositions were within the solid solution regime, i.e., below 15-0 mol% (MoO3) in V2O5. Laser Raman and XRD data indicated that solid solutions were formed, and pre- and postreaction XPS data indicated that catalyst surfaces were reduced by the 1,3-butadiene-in-air feeds. The reaction pathway for 1,3-butadiene partial oxidation to maleic anhydride was shown to involve intermediates such as 3,4-epoxy-1-butene, crotonaldehyde, furan, and 2-butene-1,4-dial. The addition of water to the reaction stream substantially increased catalyst activity and improved selectivity to crotonaldehyde and furan at specific reaction temperatures.,