The change in morphology and molecular organization occurring in bis(n-propylimido)perylene thin solid films exposed to various solvents in the gas phase were investigated using transmission and reflection-absorption infrared spectroscopy. Specifically, infrared transmission and reflection spectroscopy has been used to study the effect of solvent vapor annealing on the organization of bis(n-propylimido)perylene films. It is shown that the environmental source of molecular reorientation arises from the penetration of molecular water at the air/molecular film interface. The effect of solvent vapor annealing of several polar molecules on the orientation of bis(n-propylimido)perylene films is also reported.