Abstract The catalytic and transport cycle of the Ca-ATPase of sarcoplasmic reticulum has been described in terms of a series of reactions taking place at specific sites on its surface, each reaction being followed by relaxation of the enzyme to a new unique equilibrium configuration. The equilibrium configurations of the two phosphorylated dimeric pump units contain, in a cleft between the polypeptide chains extending into the extra-vesicular solution, a micro-aqueous phase which has a high degree of hydrogen-bonded order imposed upon it by cooperative interactions with donor and acceptor groups on its bounding surfaces. Solutes present when the phase change takes place experience a change in chemical potential which is negative for K + and other lightly hydrated ions and non-electrolytes, and becomes increasingly positive for small cations and polyphosphates as the free energy of hydration increases. The aspartyl phosphate of the first phospho-enzyme is in contact with the higly-ordered water of decreased reactivity, and is not readily hydrolysed. It can, however, react with ADP to form ATP because the overall free energy change of the reaction changes sign when all the participating species either increase or decrease in chemical potential. Ca 2+ ions bound to high affinity sites at the apex of the cleft are displaced by the more highly hydrated Mg 2+ ion, and, with relaxation to the second equilibrium configuration of the phospho-enzyme, the aspartyl phosphate becomes exposed to water of normal reactivity, and a divalent-cation specific channel opens between the monomers. Ca 2+ diffuses into the interior of the vesicle, the phospho-enzyme is hydrolysed, and with the final relaxation the channel closes and cleft water resumes bulk-phase-like properties. Calculations of changes in enthalpies and entropies of hydration with changes in solvent structure show that a decrease of the “structural temperature” of water from 25°C to 5°C is enough to account quantitatively for the mechanisms of active transport and ATP synthesis.