The present invention provides a process for preparing a co-catalyst composition of increased abrasion resistance for use in reactions which involve reaction conditions of high stress, e.g., vapor phase oxidations. The process comprises intimately mixing an aqueous suspension of colloidal silica particles with a water-soluble metal salt which is decomposable by heat to a metal oxide, calcining the mixture, adding a further amount of the aqueous suspension of colloidal silica particles, and drying this co-catalyst composition.