The redox properties of horse and yeast cytochrome c electrostatically immobilized on carboxylic acid-terminated self-assembled monolayers (SAMs) have been detd. over a broad pH range (pH 3.5-8) in the absence and presence of nitric oxide. Below pH 6, both proteins exhibit comparable midpoint potentials, coverages, and electron-transfer rate consts., which suggests that they are adsorbed on the SAM in a similar fashion. Above pH 6, a sharp decrease in electron-transfer rate consts. is obsd. for immobilized yeast cytochrome c, which is indicative of a change in the electron tunneling pathway between the heme and the electrode and hence suggests that the protein reorients on the surface. Such a decrease is not obsd. for horse cytochrome c and therefore must be related to the specific charge distribution on yeast cytochrome c. Apart from the charge distribution on the protein, the reorientation also seems to be related to the charge on the SAM surface. The presence of nitric oxide causes a decrease in electron-transfer rate consts. of both yeast and horse cytochrome c at low pH. This is probably due to the fact that nitric oxide induces a conformational change of the protein and also changes the reorganization energy for electron transfer.