Micro-Raman techniques are used to acquire Raman spectra of a water-soluble porphyrin (meso-tetrakis(p-sulfonatophenyl)porphine) adsorbed onto smooth and roughened Ag electrodes. We combine such Raman studies with cyclic voltammetric measurements conducted for the porphyrin adsorbed onto smooth, polished Ag and Pt electrodes (both as functions of pH and electrode potential) and gain insight into the mechanism for protonation of monomeric porphyrin and formation of aggregates. It is deduced that variation in the Raman spectrum, induced by potential and pH changes, is related to the interfacial proton concentration. Electrochemical reduction of water and not the release of hydrogen from a coadsorbed porphyrin leads to the formation of the diacid that then is incorporated into aggregates. Another finding is that electrochemically induced protonation and the concomitant aggregation of TSPP are reversible on a time scale defined by the diffusion of H-2 into bulk solution.