The following review explores applications of voltammetric methods for observing reactions of complex metalloproteins. Attention is focused upon the technique of ‘protein-film voltammetry’, in which the protein molecules under investigation are adsorbed on the electrode surface and electrochemically ‘interrogated.’ The experiments address a minuscule sample with high sensitivity, and optimal control over both potential and time dependence of reactions. Factors governing the voltammetric response are outlined, and particular emphasis is given to the ability to study reactions that are coupled to and may ‘gate’ the primary electron exchange processes. Examples described include proton-transfer and metal-binding reactions of iron–sulfur clusters, coupling of electron transfer in peroxidases, quantifying electron-transport pathways in multi-centred enzymes, and detection of ‘switches’ that modulate the catalysis as a function potential.