The internal friction of a series of polymethacrylates is determined by means of a computerized acoustic spectrometer between 80° and 280° K. It is found that the incorporation of tetraethylene glycol dimethacrylate as a comonomer with methyl methacrylate yields a loss peak near 180°K. This peak decreases both in strength and in temperature with decreasing divinyl content. The mechanism for this relaxation is attributed to the micro-Brownian motions of the tetraethoxy moieties. The effect of incorporating diluents in polymethacrylates is the appearance of a new “diluent peak,” which depends on the nature of the diluent. The introduction of di(ethylhexyl) adipate produces a loss peak near 140° K in the methyl, ethyl, n-propyl, and t-butyl esters of methacrylic acid. The presence of tricresyl phosphate, on the other hand, does not yield a new diluent peak. A possible mechanism for the diluent peaks consistent with these observations is the interaction of diluents with the carboxyl groups of the ester side chains.