In situ electrical conductivity measurements have been performed on vanadyl pyrophosphate samples as a function of their preparation temperature and on V-P-O catalysts as a function of their activation time. In agreement with X-ray diffraction results, electrical conductivity has shown that heat treatment of vanadyl pyrophosphates between 450 and 800 degrees C results in a less conducting state corresponding to a very crystalline form. Such a stabilized form can already be reached at a temperature between 600 and 700 degrees C. For V-P-O catalysts, electrical conductivity decreased as a function of the activation time, t(act), under the reaction mixture, reaching a steady value for t(act)greater than or equal to 84 h. Both types of solids were found to be p-type semiconductors with positive holes as the main charge carriers. During subsequent exposures to n-butane and to oxygen at reaction temperature (400 degrees C), it was confirmed that vanadyl pyrophosphate constitutes the main phase of efficient V-P-O catalysts working with some excess of surface oxygen probably in relation with the presence of V5+ species. The recorded changes in conductivity upon exposure to reactants provide illustrative evidence of a redox (Mars and van Krevelen) mechanism. The long activation period necessary to produce a high yield of maleic anhydride corresponds to structural and chemical changes of the surface which accommodates the reaction mixture and gives rise to active and selective sites able to convert n-butane to maleic anhydride.