The two-dimensional phase behavior of mixed-monolayers composed of poly(amidoamine) (PAMAM) dendrimers and n-alkylthiols adsorbed to-Au(lll) surfaces was studied using tapping mode atomic force microscopy (TM-AFM). Mixed monolayers were prepared by sequential immersion of Au(lll) substrates into ethanolic solutions of, first, dendrimers and then n-alkylthiols. Films were prepared by systematically varying the dendrimer generation (G4, G6, and G8) and the n-alkylthiol chain length (CH3(CH2)(x)SH; x = 4, 11, 15). TM-AFM was used to monitor time-dependent morphological changes in the mixed-monolayers. Dendrimer monolayers immersed in hexane solutions of n-alkylthiols do not phase separate to an appreciable extent, which demonstrates the important role of dendrimer solvation in these processes. In contrast to the amine-terminated dendrimer monolayers, thiol-terminated dendrimers do not phase separate when exposed to ethanolic n-alkylthiol solutions, suggesting that one of the driving forces for this process is the difference in adsorption energies between the amine and thiol groups and the Au substrate.