We report on the use of surface second harmonic generation (SHG) intensity measurements to understand the molecular details of potentially heterogeneous surface coverage. Using a rigid chromophore with a large second-order nonlinear susceptibility (chi((2))) and an adlayer diluent molecule with negligible nonlinear response, the incidence angle dependence of the SHG intensity was determined as a function of surface chromophore coverage. The SHG data at fixed incidence angle suggest that the average orientation of the adsorbed chromophores does not change significantly with surface coverage, resulting in the expected square-law relationship between SHG intensity and loading density. The form of the SHG data points to the presence of more than one domain in the monolayer. The data can be fit successfully using a two-domain model in which the dominant domain is characterized by chromophores oriented along the surface normal and the minority domain is comprised of a nominally random orientational distribution, with the relative amount of the disordered domain decreasing with increasing fractional chromophore coverage.