The behavior of Langmuir monolayers of octadecyl-substituted preformed polyimide molecules exhibits a strong dependence on temperature when the isotherms are measured using a Wilhelmy balance. This suggests a change in structure with temperature, though isotherms measured with a Langmuir balance change only modestly with temperature. Brewster angle microscopy images of the monolayer morphology are consistent with the notion that the monolayer is extremely rigid at 20-21 degreesC, while it has somewhat more fluidity at 28 degreesC. Polarization-modulated infrared reflection absorption spectroscopy results also indicate a qualitative difference between the characters of the monolayer at the two temperatures. The Wilhelmy plate method is sensitive to this difference, but its limited accuracy in the case of very rigid films results in a response that greatly amplifies the character of this change as compared to other means of probing the monolayer structure.