By dissolving tracer quantities of 9,10-phenanthrenequinone (PQ) in poly(methyl methacrylate) (PMMA) it was possible to study by forced Rayleigh scattering (FRS) the tracer diffusion of the unbleached PQ as well as the photoproduct which is covalently bound to the PMMA molecules. The PMMA samples with molecular weights, M-w, of 10(3)-10(5) were intermittently annealed at 80 degrees C for diffusion times of up to one year. From the results we conclude that polymer chain diffusion may be possible at temperatures more than 20 K below the glass transition temperature T-g, but further experiments are necessary, in particular, since we cannot exclude that our results are due to yet unexplained long time aging effects in PMMA. The translational diffusion coefficients of the unbleached PQ are enhanced by several decades in comparison with predictions from rotational correlation times that were determined by using a modified FRS technique.