The moderation of subexcitation electrons in H2 and D2 is investigated by using the Spencer–Fano (SF) equation and the continuous‐slowing‐down approximation (CSDA). Cross‐section data are adopted from the compilation by Buckman and Phelps [J. Chem. Phys. 82, 5001 (1985)]. Throughout, we focus on isotope effects, i.e., differences between H2 and D2, and consider electrons at energies appreciably higher than thermal energy. In summary, because vibrational and rotational excitation channels have lower thresholds in D2, subexcitation‐electron behavior in D2 at energies below 0.6 eV shows different characteristics from that in H2. The moderation rate in H2 is larger than that in D2 by a factor of about 1.7. This difference is close to but not exactly the factor of 2 expected from an elementary estimate.

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