We report a solvent-induced phase transition in pentacene thin films, from a “thin film” phase to a bulk-like phase. X-ray diffraction indicates that as-deposited thermally evaporated pentacene films consist mainly of (001)-oriented pentacene with an elongated (001) plane spacing of 15.5±0.1 Å, and a minor amount with a (001) plane spacing of 14.5±0.1 Å. When such films are exposed to solvents such as acetone, isopropanol, or ethanol, the plane spacing of the entire layer shifts abruptly from the elongated (001) plane spacing to the bulk value and this shift is accompanied by a macroscopic change in film morphology. While molecular ordering is maintained as indicated by x-ray diffraction, thin film transistor performance is severely degraded, most likely as a result of the morphological changes in the film. © 1999 American Institute of Physics.