SERS-based spectroelectrochemistry was used to investigate interfacial structural properties of mixed self-assembled monolayers (SAMs) as a function of their composition. Mixed monolayers were formed from ethanolic solutions containing 12-mercaptododecanenitrile and n-heptanethiol. The ratio of the two-component concentrations in solutions from which the SAMs were assembled was varied to provide a series of SAMs with varying surface concentrations of the terminal nitrile group. The intensities of Raman scattering from vibrational modes associated with the nitrile and methyl termini were used to determine the relative composition of the mixed monolayer versus component concentrations in the assembly solutions. Stark tuning of the nitrile group served as a localized probe of the interfacial environment. Stark tuning rates of the nitrile probe molecule exhibited a dependence on monolayer composition comprising two distinct regions: Stark tuning rates at high nitrile surface coverages were large and did not exhibit significant composition dependence, while Stark tuning at low nitrile coverage was weaker and varied with SAM composition. Raman scattering obtained in the C-H stretching spectral region is modeled using ab initio electronic structure calculations and used to determine conformational order within the SAM as a function of applied potential.