Abstract Rat-liver mitochondria, devoid of outer membrane, were subjected to the action of pure phospholipase A 2 from porcine pancreas. Degradation products were removed by treatment with bovine serum albumin. Arrhenius plots of succinate-cytochrome c reductase and cytochrome c oxidase activities revealed two breaks, in the temperature region 10–30 °C. The exact values of the break temperatures were dependent on the extent of phospholipid hydrolysis, particularly in the case of succinate-cytochrome c reductase. The higher temperature break probably reflects a phase change of the lipids in close proximity to the enzyme molecules, while the lower temperature break can possibly be explained by a reorganization of the lipids in the gel phase. The values of the apparent activation energies varied with the extent of phospholipid hydrolysis and, consequently, with the relative phospholipid composition, indicating that the magnitude of the activation energy of a lipid-dependent enzyme is not merely governed by the physical state, but also by the composition of the lipid environment. Differences were observed in the effect of phospholipase treatment between the two enzyme activities studied. This suggests a certain degree of heterogeneity in the distribution of the lipids which are directly involved in the functioning of the enzyme molecules.