The diffusion coefficients of a photochromic dye, TTI, and of methyl methacrylate, MMA, have been measured in MMA/poly (methyl methacrylate) (PMMA) solutions by forced Rayleigh scattering and field-gradient NMR, respectively. The polymer concentration was varied from 0 to 80% by weight, and the temperature from -10 to +70 °C. The TTI data could be reduced to values representative of MMA diffusion by taking into account the different sizes of the two molecules. The solution glass transition temperature of MMA/PMMA solutions was also measured between 70 and 100 % polymer. The MMA diffusion data are compared to available data and estimates of the propagation rate coefficient and of the initiator efficiency over the same polymer concentration range. Contrary to the general assumption, it is found that propagation is not diffusion controlled. The measured decrease of the propagation rate coefficient requires an explanation different from the one that has traditionally been offered. The comparison between MMA diffusion and initiator efficiency supports the idea that the latter is determined by the mobility of the radical fragments. These observations combined still provide an explanation for the reduction in the overall rate of polymerization, Rp, observed during bulk polymerizations of MMA carried at intermediate to high conversions. The decrease in Rp observed in emulsion polymerization, however, cannot be similarly interpreted. The polymer concentration, wpg, at which an MMA/PMMA solutions turns glassy is compared to the limiting conversion. The limiting conversion correlates well with wpg, but the reason for this is uncertain in light of the above findings.