Molecular ruler surfactants, solvatochromic probes of solvent polarity, have been used to examine changes in solvent polarity across weakly associating liquid/liquid interfaces. The water/alkane interfaces were formed between an aqueous subphase and either cyclic (cyclobexane and methylcyclohexane) or linear (octane and hexadecane) alkanes. Resonance-enhanced second-harmonic generation was used to collect effective excitation spectra of species adsorbed to these interfaces. As surfactants lengthened, the surfactant probe sampled an increasingly nonpolar environment as evidenced by an excitation wavelength that shifted toward the alkane limit. Data suggest that all four water/alkane interfaces are molecularly sharp (<9 Angstrom), but that differences in the solvent molecular structure alter the transition from aqueous to organic solvation across the interface. Polarity across two interfaces (cyclohexane and hexadecane) changes gradually over the distance spanned by ruler surfactants. In contrast, the transitions at the interfaces between water and methylcyclohexane and octane appear much more abrupt. These findings appear to correlate with each organic solvent's ability to pack and associated free volume. More free volume in the organic phase leads to a more abrupt water/alkane interface. Results are interpreted on the basis of recent molecular dynamics simulations examining polarity at different water/monolayer interfaces.