Competitive self-assembly of ferrocenylhexanethiol (Fc-C6-SH) and 1-alkanethiols onto gold from ethanol and 1-hexanol solution is interpreted using a model based on coupled Frumkin isotherms. Alkanethiols with long alkyl chains are always preferentially adsorbed relative to Fc-C6-SH. In the limit of low ferrocene coverage, that preference is described quantitatively in terms of a parameter DELTA(DELTAG2 - DELTAG1)' that describes the incremental change per methylene unit in the characteristic free energy difference controlling competitive adsorption. That incremental change is found to be -1.9 kJ mol-1 per methylene unit increase in alkanethiol chain length for adsorption from ethanol and -0.8 kJ mol-1 per methylene unit increase for adsorption from 1-hexanol. These values are smaller than would be anticipated from independent estimates of the incremental change in cohesive energy of n-alkanes per methylene unit derived from vapor pressure, fusion enthalpy, vaporization enthalpy, and sublimation enthalpy data. Finally, data on ferrocene coverage as a function of exposure time to a coating solution indicate that monolayer composition can change over time during self-assembly, reaching a limiting composition only after exposure times of between several hours and several days. It is postulated that the preference for adsorption of long-chain alkanethiols is manifest primarily in the slow relaxation of an initially-formed disordered monolayer to a more ordered structure.