Establishing control of molecular order, orientation, and the size of crystalline domains within thin films is crucial to engineering physical and charge-transport properties. Vapor-deposition polymerization of polyimides is accomplished by controlling incident molecular flux of sublimed dianhydrides and diamines. Crystallites containing unreacted monomer are observed when monomer fluxes are unbalanced. Diamine crystallites nucleate at early stages of deposition, grow radially during film growth, and are smaller at high deposition rates. Diamine crystallites protrude above the film, suggesting that monomer surface diffusion is significant. This high surface mobility likely promotes polymerization and explains other phenomena observed in vapor deposited films.