The average ionic charge created in Ar, Kr and Xe as a result of X-ray photoabsorption has been determined using time-of-flight mass spectroscopy in conjunction with synchrotron radiation (XTOFS). Measurements were carried out at the Ni K edge, 1.5 Å. Calculations predicting the degree of electron multiplication expected from inelastic collisions in the gas at atmospheric pressures have been performed. Theory is found to be in agreement with present experimental findings. The estimated total electron yield in the gas is a guide in the determination of the efficiency of a gas proportional counter. Xenon gas is found to be the preferred candidate.