The molecular description of the mechanism of F₁–ATPase is based mainly on high-resolution structures of the enzyme from mitochondria, coupled with direct observations of rotation in bacterial enzymes. During hydrolysis of ATP, the rotor turns counterclockwise (as viewed from the membrane domain of the intact enzyme) in 120° steps. Because the rotor is asymmetric, at any moment the three catalytic sites are at different points in the catalytic cycle. In a “ground-state” structure of the bovine enzyme, one site (β_E) is devoid of nucleotide and represents a state that has released the products of ATP hydrolysis. A second site (β_TP) has bound the substrate, magnesium. ATP, in a precatalytic state, and in the third site (β_DP), the substrate is about to undergo hydrolysis. Three successive 120° turns of the rotor interconvert the sites through these three states, hydrolyzing three ATP molecules, releasing the products and leaving the enzyme with two bound nucleotides. A transition-state analog structure, F₁–TS, displays intermediate states between those observed in the ground state. For example, in the β_DP-site of F₁–TS, the terminal phosphate of an ATP molecule is undergoing in-line nucleophilic attack by a water molecule. As described here, we have captured another intermediate in the catalytic cycle, which helps to define the order of substrate release. In this structure, the β_E-site is occupied by the product ADP, but without a magnesium ion or phosphate, providing evidence that the nucleotide is the last of the products of ATP hydrolysis to be released.