The influence of annealing (thermal treatment) in different atmospheres on the conductivity and dielectric properties of As-diluted lead phosphate Pb3(P1–xAsxO4)2 was studied. The anomalous low frequency dielectric behavior of these crystals was found to be related to their conductive properties. A model invoking the dissociation of polar impurity complexes is proposed in order to explain the observed giant dielectric maxima and their strong frequency dispersion. It was found that extrinsic proton conductivity dominates the dielectric properties in the temperature range 350–900 K in Pb3(P1–xAsxO4)2 and that annealing in wet atmosphere increases the protonic conductivity by more than one order of magnitude.