The experimentally measured effect of the odd magnetic isotope 25Mg on the rate of ATP synthesis by the mitochondrial F0F1 enzyme is used to compare the rates of the main reactions in the catalytic site, in the framework of a radical-ion process. The rate-limiting step of synthesis is the addition of the ADP oxyradical to the phosphate P=O bond. The relationship of the rate constants deduced from the magnetic isotope effect shows that the mechanochemical efficiency of ATP synthase operating with spinless 24Mg or 26Mg nuclei will not exceed 50%. The performance is nearly doubled with magnetic 25Mg.