The calibration of a cobalt-60 beam in a phantom with an ion chamber, which has been calibrated with respect to exposure, requires the use of a displacement correction factor which essentially corrects the photon fluence for the attenuation and scatter when the chamber with buildup cap is removed and replaced by phantom material. To determine the displacement factor, Ac, a special set of cylindrical ionization chambers with various volumes were constructed out of polystyrene. Tissue-air ratios were measured with these chambers for cobalt-60 gamma rays in a polystyrene phantom, and the ratio Ac/Aeq was experimentally determined. In order to calculate Ac from this ratio, Aeq was determined also. It was found that Ac depended on chamber diameter only, and not on field size or depth. A value of 0.990 for Aeq is recommended and a table of Ac for chambers of different outer diameters is included.