A phosphorus-31 solid echo chemical shift anisotropy line shape study of tris(2-ethylhexyl) phosphate in tetramethyl polycarbonate determined the rate and amplitude of diluent motion in this mixed glass as a function of temperature and concentration. The dynamics of the diluent are observed to be bimodal with diluent in contact with other diluent on a random basis, displaying much greater mobility than diluent surrounded by polymer. A lattice model adequately accounts for the population of each type of diluent. The diluent in contact with other diluent considered to be a microcluster undergoes isotropic Brownian rotational diffusion with an apparent activation energy of 56 kJ/mol. The isolated diluent molecules undergo Brownian rotation restricted to a cone, and the rate and amplitude of this motion increase slowly with temperature. The temperature and breadth of the mechanical loss peak at a frequency of 1 Hz associated with rotation of the diluent in microclusters can be calculated from the interpretation of the line shape data. It is predicted to occur at a temperature of -56 degrees C, which Lies on the low-temperature side of the polymer loss peak and may account for some of the increased mechanical loss peak at lower temperatures upon addition of diluent. The mechanical loss peak is little changed in amplitude upon addition of diluent, in contrast to significant suppression observed in bisphenol A polycarbonate. The coupling model of Ngai and Yee accounts for this difference in terms of the inherent rate of the sub-glass transition motion of the two polymers relative to the inherent rate of the motion of the diluent. The size of the microclusters was determined from proton spin diffusion to be 12 Angstrom and to contain about two diluent molecules, in agreement with the lattice model description.