A total of 29?hitherto unknown vanadyl(IV)-, vanadyl(V)-, and vanadate(V) phosphates were synthesized, structurally characterized, and tested in terms of their behavior as catalysts in the selective oxidation of n-butane to maleic anhydride. The new materials did not surpass the performance of commercially used (VIVO)2P2O7-based catalysts, but a silver vanadium phosphate of approximate composition Ag2VIV,VP1.6O7+d (0 = d = 0.5), supposedly having a vanadyl(IV,?V) phosphate pyrophosphate layer structure, shows promising activity and selectivity with potential for further improvements. With the aim to study the charge carrier dynamics a new method for in situ non-contact measurement of the electric conductivity of catalysts was developed. The analysis of conductivity changes in response to the chemical potential of oxygen in the gas phase offers valuable clues to understand structure-reactivity relationships in selective oxidation catalysis.