The interfacial behavior of a variety of proteins ranging in molar mass from 2500 to 340 000 g mol(-1) was studied at a Pt electrode surface at 298 K using the electrochemical quartz crystal nanobalance (EQCN) technique of simultaneous measurements of frequency and cyclic voltammetry (CV). It was shown that the EQCN frequency measurements did not directly monitor the molar mass of the adsorbed protein at anodic potentials but instead measured changes in the surface oxide in the absence and presence of adsorbed protein. However, at a potential characteristic of the double layer for platinum, EQCN frequency measurements gave a measure of the extent of solvent displacement by the adsorbed protein. The values for the Gibbs free energy of adsorption, DeltaG(ADS), obtained with these EQCN frequency measurements gave excellent agreement within experimental uncertainty with those obtained from the simultaneous CV measurements for all proteins studied. The adsorption process was modeled using the Langmuir adsorption isotherm. The smallest molecules studied, chain A and chain B of insulin, have the lowest affinity for the platinum surface as indicated by their small negative DeltaG(ADS) values, while the larger proteins, such as fibrinogen, have the greatest affinity for the platinum surface.