The mechanism of alkali ion movement in glasses during electron‐probe analysis must include a dependence on the temperature of the volume irradiated. Experimental results of the composition changes of binary oxide glasses of Na2O and K2O with SiO2during electron bombardment indicate that there is a critical temperature for alkali ions in the diffusion process. Dependence of the critical temperature on composition and a comparison of these temperatures to the equilibrium temperature that can be produced from heat flow consideration indicates that for small beam diameters the actual temperature may be lower than that predicted. The rate of change of temperature may be more important to a probe operator than the equilibrium temperature.