Formal potentials for ferrocene in self-assembled monolayers of N-(7-mercaptoheptyl)ferrocenecarboxamide coadsorbed with n-alkanethiol derivatives of variable chain length and terminal functionality are substantially more positive than the corresponding potentials for the same ferrocene compound in bulk solution. The differences in formal potential are a strong function of the chain length and terminal functionality of the alkanethiol coadsorbate, the nature and concentration of supporting electrolyte, the coverage of ferrocene on the electrode, and the solvent. Two physical models for the electrode/monolayer/solution interface are invoked to explain these differences. One model is based on ion solvation energetics in the interfacial microenvironment relative to that in bulk solution and describes essentially a solvent effect on the formal potential for the immobilized redox-active moieties. The other is based on the spatial distribution of ions in the interfacial region and describes essentially a double-layer effect on the apparent formal potential for the immobilized redox-active moieties. Quantitative predictions are developed from these models that specifically address the effects of electrolyte type and concentration, solvent, ferrocene surface coverage, and coadsorbate chain length. It is concluded that both interfacial solvation and ion spatial distribution effects must be considered to adequately explain the data.