The effect of a magnetic field on the oxidation of hypophosphite ion catalysed by electrolessly deposited nickel in aqueous deuterium oxide solution was studied. The steady rate of gas evolution was affected by both the magnetic field and the concentration of deuterium oxide. Such an effect was due to the magnetic spin rephasing of a radical pair of atomic hydrogen and atomic deuterium. The ratio of fractions of active sites occupied by atomic hydrogen and deuterium increased with magnetic flux density, but decreased with the mole percentage of deuterium oxide. The results indicated that the recombination rate of the radical pair of atomic hydrogen and atomic deuterium was affected by the magnetic field. A reaction mechanism of the oxidation of hypophosphite ion based on adsorption was proposed, and a steady rate equation was obtained experimentally. Based on the results of the steady rate, an empirical rate equation was obtained for the magnetic flux density B in the range 0.15-0.30 T, r(s) = 6.20(0.965 x exp(5.62B)[H2O]/[D2O])/(1 + 0.965 exp(5.62B)(H2O]/[D2O]) + (2.92 exp(1.46B))/(1 + 0.965 x exp(5.62B)[H2O]/[D2O]).