The involvement of protons in the heterogeneous electron transfer between cytochrome c and a gold electrode to which it is attached was studied by comparing the electron transfer rate constants for H2O and D2O solutions. Rate constants were measured as a function of the electrochemical cell solution and the protein incubant solution, i.e., k(0)(incubant, cell). Two separate isotope effects exist: a cell "isotope effect", KIEcell = k(0)(H2O, H2O):k(0)(H2O, D2O), which is manifest at short time scales (<30 s) and arises from the viscosity difference between H2O and D2O, and an incubant isotope effect, KIEinc = k(0)(H2O, H2O):k(0)(D2O, H2O), which is manifest at longer times (> 2 h) and results from H/D exchange. The two isotope effects are approximately equal (similar to 1.2) and a total isotope effect KIEtotal = k(0)(H2O, H2O):k(0)(D2O, D2O) can be constructed that is the product of KIEcell and KIEinc. The nature of the electron transfer process, possible coupling to a proton transfer process, and the involvement of specific hydrogens in the transfer mechanism are discussed.