The role of mass transfer effects in degradation of carbon fiber reinforced bismaleimides was examined utilizing the concepts of Thiele modulus and effectiveness factor. Unidirectional samples were aged at 250, 260, and 270 degrees C. The weight loss rates from the three types of composite surfaces (resin covered surface, 0 degrees surface, and 90 degrees surface) were obtained from the weight loss data of different sample geometries. As observed previously, the weight loss rate from the resin covered surface showed a typical behavior for a reaction that becomes increasingly diffusion limited. The effective reaction and diffusion coefficients were obtained based on the initial and final weight loss rates, and activation energies for reaction and diffusion were also determined. The activation energy for diffusion was an order of magnitude higher than customary, indicating that phenomena other than diffusion (increase in effective surface area due to cracking) also contributed to the weight loss rate. Overall, this work illustrated the applicability of an unreacted core type model to composite degradation and the importance of accounting for the anisotropy as well as mass transfer effects in composite degradation.