The pH2 conversion rate has been compared with that for oD2 and H2+ D2→ 2HD on nickel wires rigorously cleaned in vacuo of 10–10 torr. The rates are significantly higher than on evaporated films, prepared under similar conditions but still possibly contaminated. The results at 1 torr fall into three temperature ranges (a) 330–400°K, a fractional order (ca. 0.6) reaction, activation energy 5–7.5 kcal mole–1. There is evidence for a zero order H2+ D2 reaction setting in as the pressure is lowered to 0.3 torr; (b) 200–300°K, a fractional order reaction, 0.05–0.40, with a lower activation energy, 1.5–2.7 Kcal mole–1. In (a) and (b), pH2, oD2 and H2+ D2 rates are very similar to each other. On decreasing the temperature to (c) 77–200°K, the activation energies for the pH2 and oD2 rates decrease to zero, so that at 77°K these two reactions go 103 times faster than H2+ D2. The mechanisms advanced are for (a) Bonhoeffer-Farkas, (b) Eley-Rideal, and (c) a paramagnetic conversion induced probably by unpaired d electrons in the Ni and proceeding through a Harrison-MacDowell mechanism.