Forced Rayleigh scattering was employed to measure diffusion coefficients for a ternary system comprising camphorquinone or diacetyl in poly(methyl methacrylate) with methyl isobutyrate as the diluent. Measurements were made at polymer weight fractions, w(p), over the range 0.78-0.90 and temperature 25-50 degrees C for camphorquinone (which spans the glass transition regime), and at w(p) = 0.9 and 0.95 and at room temperature for diacetyl. Results for camphorquinone show a strong temperature dependence, with diffusion coefficients changing as much as two orders of magnitude between 25 and 50 degrees C, and showing high activation energies for diffusion approximate to 125 kJ mol(-1). No sharp change in the activation energy for diffusion was observed as the glass transition is crossed. The pre-exponential factor for the diffusion coefficient decreased dramatically through the glass transition. The diffusion coefficients of small penetrants in glassy polymers appear strongly dependent on the molecular size. (C) 2000 Elsevier Science Ltd. All rights reserved.