We present an electron spin resonance (ESR) study of Cs2CO3 and LiF doped tris(8-hydroxyquinoline) aluminum (Alq(3)) thin films. The g values of the Cs2CO3 and LiF doped Alq(3) thin films were obtained as 2.0040 and 2.0028, repectively. The g value of the LiF doped film was very close to a theoretically calculated one for an isolated Alq(3) radical anion, while Cs2CO3 doped film showed a large g shift. This feature can be explained by large spin-orbit interaction caused by heavy Cs atoms. While doping of Cs2CO3 did not showed cathode-metal (Al and Au) dependence, LiF doping required Al as cathode.