Catalysts based on silver vanadium phosphates with a general chemical formula Ag2VPxOd (1 <= x <= 2; 6 <= d <= 8) have been prepared and tested in gas phase oxidation of n-butane. This general chemical formula comprises the two known phosphates Ag-2((VO2)-O-V)PO4 (x = 1; d = 6) and Ag-2((VO)-O-IV)P2O7 (x = 2; d = 8) and one novel compound with an approximate composition (Ag2VP1.6O7+delta)-P-IV,V (0 <= delta <= 0.5). Among the prepared compounds, Ag-2((VO2)-O-V)PO4 shows the highest activity while Ag-2((VO)-O-IV)P2O7 is completely inactive. The novel phase (Ag2VP1.6O7+delta)-P-IV,V (0 <= delta <= 0.5) is the only compound with selectivity to maleic anhydride. A structure-property relationship for the prepared catalysts is discussed.