The influence of exposure to a wet atmosphere and vacuum annealing effects on the electron and hole traps in an anode oxide of GaAs by using "optical charge method" have been studied. Charging of the sample surface by illumination for photon energies of 1.0 to 5.4 eV was recorded from the change in the dark value of the contact potential difference (CPD) between the sample and a transparent nickel net electrode. The CPD was measured using a dynamic capacitor (Kelvin) method. From correlation of the optical charging and mass spectrometry and electron spin resonant method experimental data it has been found that the defects which include arsenic vacancies are traps for the holes. On the other hand, the oxygen atoms localized on gallium vacancies in the anode oxide are responsible for the electron trapping.