The application of cavity‐ionizaion chambers to the standadization of60Co γ‐ray beams, in terms of exposure, requires that the specific ionization of airJg, be corrected for the attenuation and scatter of the incident rays by the wall, central electrode, and suporting stem of the chamber. A Monte Carlo photon–electron transport code has been developed for the purpose of calculating this correction for spherical and cylindrical chambers. The code has been applied to a spherical graphite chamber having dimensions typical of the chambers used by the NBS, the calculated wall‐correction factor is in close agreement with the average of the NBS factors which were determined experimentally. The code was also used to calculateAeq, which is central to the determination of tissue‐air ratios. The calculated value, 0.989±0.003, is very close to the generally accepted value, 0.985.