Understanding the factors that affect molecular self-assembly is crucial to building designed nanoscale structures. We have deposited nanoscale films of polyhedral oligomeric silsesquioxane (POSS) onto polished silicon substrates from a range of organic solvents. We studied these films using synchrotron X-ray reflectivity and found that dip-coating from benzene, toluene, or chloroform results in near-substrate ordering only, but when acetone, hexane, or THF is used, self-assembled layers are formed throughout the entire deposited film. We conclude that solvent polarizability is the factor that determines the alignment of the POSS molecules. We have successfully tested this prediction using additional solvents selected on the basis of their calculated polarizabilities.