This review focuses on the electrochemical and spectroelectrochemical studies that gave insight into redox potentials of the four mitochondrial complexes and their homologues from bacterial respiratory chains using O-2 as a terminal acceptor, thus providing crucial information about their reaction mechanism. Advantages and limitations of the use of the different techniques for the study of membrane proteins are presented. Electrocatalytic experiments are described that revealed specific features of the reaction with the substrates and inhibitors. An overview is given on the great variability of the redox and catalytic properties of the enzymes in different organisms that may be due to adaptation to the specific environments in which these enzymes function. The adaptation of the redox chain to the different types of quinone and substrates is analyzed, and future studies are discussed.