The kinetic behaviour of heavy meromyosin (HMM) and subfragment-1 (S-1) has been compared for a range of pH and temperature for normal and SH-1 modified proteins. No significant differences were found between S-1 and HMM and the evidence is consistent with an identical independent site mechanism of the form first proposed by Bagshaw and Trentham: $$\begin{gathered} M + ATP\mathop { \leftharpoondown \rightharpoonup }\limits^{K_1 } M.ATP \mathop { \leftharpoondown \rightharpoonup }\limits^{k_2 } M.ATP^* \mathop { \leftharpoondown \rightharpoonup }\limits^{k_3 } M.Pr^{**} \mathop {\mathop { \leftharpoondown \rightharpoonup }\limits_{H^ + ,P_i } }\limits^{k_4 } \hfill \\ M.ADP^* \mathop { \leftharpoondown \rightharpoonup }\limits^{k_5 } M.ADP \mathop { \leftharpoondown \rightharpoonup }\limits^{K_6 } M + ADP \hfill \\ \end{gathered} $$ where asterisks refer to states of enhanced tryptophan fluorescence and a M.Pr is a state in which both products are bound.k3 andk5 had a large temperature dependence (Arrhenius activation energy approximately 100 kJ mol−1).k3 andk5 increased with increasing pH and the variation fitted a titration curve with pK of 7.4. The rate of step 4 was measured by decay of tryptophan fluorescence, proton release and phosphate release measured as an increase in conductance. The temperature dependence curves fork4 andk5 cross over at 11–13° C for Mn2+-ATPase, 3–5° C for SH-1 modified Mg2+-ATPase and below 0° C for normal Mg2+-ATPase. A change of the rate-determining step fromk4 tok5 accounts for the nonlinear Arrhenius plots of Mn2+-ATPase and SH-1 modified Mg2+-ATPase. Modification also reduced the rate of the hydrolytic stepk3 measured by the maximum rate of the phosphate early burst. The two fluorescence transitions (steps 2 and 3) are easily resolved for the modified enzyme andk3 measured by fluorescence was equal to the rate of hydrolysis obtained from phosphate measurements.