Surface photovoltage spectrum (SPS) and field-induced surface photovoltage spectrum (FISPS) are valuable techniques for the determination of pn conduction type of organic semiconductors. Here, conduction characters of azo pigment films composed of pure 4,4'-2-[N-(beta-hydroxyl)naphthyl]azodiphenyl (A) and 4,4'-2-{N-[beta-hydroxyl-gamma-(p-nitroanilino)naphthyl]}azodiphenyl (B) as well as A doped with I-2 vapor (AI(2)) and B doped with NH3 gas (BNH3) were explored by means of these techniques. The results of the exploration show that film A has n-type character and film B p-type character. On the basis of the reverse photovoltaic properties, films AI2 and BNH3 were judged to be characteristic of p-type and n-type conduction, respectively. It appears that a group rich in electrons, either an electrophobic substituent (naphthanol) at the end side of the azo pigment molecule (A) or a dopant (NH3), bestows the azo pigments with n-type character, whereas an electrophilic group, such as the substituent (p-nitroanilino) connected to azo pigment molecules (B) and I-2, gives them p-type character. The phenomena are fully discussed based on the energy band modes of the Fermi levels of these materials.