In many experimental studies with incubated mitochondria it appeared that atstate 4, i.e., in the presence of oxygen and oxidizable substrates but in the absence of added ADP, oxidative phosphorylation behaved almost like a system at thermodynamic equilibrium. This conclusion was based exclusively on the comparison of the measured ∆G values of the driving reaction (oxidation) and the driven reaction (phosphorylation) with their known equilibrium values. From this it was then assumed that the efficiency of mitochondrial energy transductions, notably oxidative phosphorylation, can be treated within the framework of classical equilibrium thermodynamics. Consequently, oxidative phosphorylation at state 4 appeared to operate with an efficiency of almost 100%. Furthermore, it was tacitly assumed that this thermodynamic treatment of oxidative phosphorylation can be extended to mitochondria incubated in the presence of ADP, i.e., under state 3 conditions. Consequently oxidative phosphorylation was assumed to operate with nearly 100% efficiency also at state 3.