By using mixtures of molecules which are sterically similar but contain functional groups with differing electronic properties, we have obtained scanning tunneling microscopy (STM) images of domains of randomly mixed monolayers of organic molecules on graphite, and the different molecules could be distinguished by STM. It was determined that molecules in the centers of the domains could still exchange readily with the solution, something which could not be determined using monolayers with single-component domains. The typical residence time of a molecule in the monolayers appeared to be between 0.2 and 20 s, and some variation was observed for different mixtures. We have also shown one case in which the composition of the adsorbed monolayer appears to be nearly independent of the composition of the bulk solution over almost 3 orders of magnitude. Although the mixture components could often be distinguished by STM, other times little or no difference was observed, even when image resolution appeared to be quite good in general. This suggests that the factors leading to STM contrast in organic molecules are complex, and the quest for reliable functional group identification by STM may be more difficult than expected.