The surface photovoltaic properties of four azo pigments(A, B, C, D) were studied by surface photovoltage spectra (SPS) and field induced surface photovoltage spectra (FISPS). The SPV responses of samples A and D were all enhanced when a negative electric field was applied and were all weaken when a positive voltage was applied. It shows that samples A and D indicate n type characteristics. On the contrary, samples B and C indicate p type characteristics. Combining with phase technique and molecular structure of azo pigments, the effect of group on Fermi energy level and conduction type of azo pigments was discussed.
