OXIDE catalysts, which are used in a broad range of important industrial proccsses(1-4), are generally prepared in the form of a precursor which is converted into the active catalytic form only under well defined reaction conditions. A vanadium phosphorus oxide catalyst is used to effect selective oxidation of n-butane to maleic anhydride : (VO)(2)P2O7 is considered(5-7) to be the main active component, and is prepared from the precursor VOHPO4.O.5H(2)O. Here we use in situ Raman spectroscopy to study the conversion of this precursor to the active form in real time. We find that, during this process, the crystalline structure of VOHPO4.O.5H(2)O becomes totally disordered at the same time as selectivity for maleic anhydride becomes apparent. Furthermore, our results indicate that this Product seems to play a role in bringing about this change; thus, it seems that the selective reaction product assists in the creation of the active sites. We suggest that this phenomenon might be quite general in oxide catalysis.