The polyimide HFPE-II-52 was developed at NASA Glenn Research Center for use as a matrix in high temperature composite materials. The unique properties of such composites stem largely from the performance of the matrix at high temperature. Thus, as part of a larger effort to Study high temperature composite materials, the linear viscoelastic properties of HFPE are measured and a mathematical model of the properties is developed. In particular, storage, loss, and stress relaxation moduli were obtained from cyclic and transient loading experiments. A Prony series was fit to the relaxation modulus data. As a cross check, the fit to the relaxation modulus was converted to storage and loss moduli and compared with those measured directly. Effects of postcuring and of moisture on the properties are investigated as well. These results provide researchers with a constitutive model for HFPE-II-52 and provide some insight into the performance of HFPE matrix composites at high temperatures. (c) 2006 Wiley Periodicals, Inc.