CELL respiration in eukaryotes is catalysed by the mitochondri`al enzyme cytochrome c oxidase. In bacteria there are many variants of this enzyme, all of which have a binuclear haem iron-copper centre at which O-2 reduction occurs, and a low-spin haem, which serves as the immediate electron donor to this centre(1). It is essential that the components of the cell respiratory system have a high affinity for oxygen because of the low concentrations of dissolved O-2 in the tissues; however, the binding of O-2 to the respiratory haem-copper oxidases is very weak(2,3). This paradox has been attributed to kinetic trapping during fast reactions of O-2 bound within the enzyme’s binuclear haem iron-copper centre(2). Our earlier work(3) indicated that electron transfer from the low-spin haem to the oxygen-bound binuclear centre may be necessary for such kinetic oxygen trapping. Here we show that a specific decrease of this haem-haem electron transfer rate in the respiratory haem-copper oxidase from Escherichia coli leads to a corresponding decrease in the enzyme’s operational steady-state affinity for O-2. This demonstrates directly that fast electron transfer between the haem groups is a key process in achieving the high affinity for oxygen in cell respiration.