The effect of water vapor on a polyimide has been investigated in the temperature region of −190 to 325°C using an automated torsion pendulum. A damping peak, T developes at −122°C (1 Hz) in the thermomechanical spectra of the polymer which is due to water–polymer interactions. The peak intensifies as the humidity of the conditioning atmosphere, at both 30° and at 325°C, increases and attains a limiting value at about 3000 ppmv H2O. The process is reversible. It is concluded that the number of sites in the polymer for interaction with water is low and limited (<0.3 molecule H2O per polymer repeat unit) and, assuming direct proportionality between the intensity of the loss peak and the amount of water adsorbed, that the adsorption of water vapor follows the Langmuir isotherm.