In stationary glow plasmas, thermal equilibrium of the electron gas with the neutral gas is established, if, with increasing gas pressure, the diffusion lifetime of plasma electrons becomes high enough. In D2-plasmas, thermal equilibrium sets in at higher pressures than in H2. Equal electron temperatures are measured in H2- and D2-glows, if the corresponding gas pressures are in the ratio p d 2/ P h 2=1.22. This isotopic effect of electron temperature and of thermal equilibrium is compared with calculations of the Boltzmann equation, taking into account the influence of ambipolar diffusion on the electron velocity distribution. There is a good agreement between measurement and theory.