The effect of temperature on the hydrolase activity of mitochondrial pyrophosphatases, i.e. soluble (PPase I) and membrane (PPase II), has been studied. In contrast to the soluble species, the membrane form has inflexions in the Arrhenius curves. However, after lipidization of PPase I by various phospholipids, the curves also acquire inflexion points, which prove similar or identical with the phase separation points of the lipids used. The closeness of the inflexion points of PPase II, containing phosphatidylcholine, and PPase I lipidized by mitochondria phosphatidylcholine, to the phase separation points of this lipid indicates that the inflexions on the PPase II curves should be ascribed to this phospholipid. It has been shown that the hydrolysis of PPi by SMP is affected by the cooperative rearrangements of the entire lipid component of the membrane rather than by the change of the phase state of PPase II phosphatidylcholine. Reconstitution experiments on the PPi synthesis system have shown that after lipidization PPase I is able to incorporate into SMP and become a coupling factor for respiration and PPi synthesis, like PPase II.