Polymer mixtures are investigated by studying the properties of a dilute probe polymer (polystyrene, PS) in the presence of a matrix polymer (poly(methyl methacrylate), PMMA) and a small molecule solvent. The solvent is chosen to be good for both polymers, and the concentration ranges are such that no phase separation occurs. Measurements include total intensity and dynamic light scattering, and viscometry. In the scattering experiments, a contrast-matching technique allows the study of the probe while in the presence of the matrix polymer. Specific points of interest are the effects due to the small positive interaction parameter x between PS and PMMA and the importance of the ratio of probe and matrix molecular weights. In addition, the interpretation of changes in the initial dependence of the mutual diffusion coefficient on probe concentration, kd(app), with matrix concentration is emphasized. By comparison of the data from ternary solutions with results for binary solutions it is found that x has little or no effect on the cross Huggins coefficient or on the radius of gyration of the probe but does have a measurable influence on the mutual diffusion between the probe and the matrix. Comparison of the contraction of the radius of gyration of a probe (Mp = 9.3 × 105) as a function of matrix concentration for three matrix polymers varying widely in molecular weight (MM = 1.3 × 106,7.0 × 104,7.0 × 103) gives insight into the molecular weight ratio for which the small matrix chains penetrate the domain of the larger probe coil in dilute solution. Apparently, there is little interpenetration between the probe and the 7.0 × 104 matrix polymer for cp* < Cm < cm*, whereas there is substantial interpenetration with the 7.0 × 103 matrix polymer over a similar range of matrix concentration; cp* (cm*) denotes the overlap concentration for the probe (matrix). The initial probe concentration dependence of the mutual diffusion coefficient between the probe and the matrix, kd(app), has been determined as a function of matrix polymer concentration for three pairs of probe and matrix polymers (9.3 ×105/8.4 × 105, 2.3 × 105/8.4 × 105, and 2.3 × 105/6.6 × 104, respectively) and is interpreted in terms of the thermodynamic and frictional influence of the matrix polymer. The thermodynamic effect dominates at low concentrations of matrix polymer, and the frictional influence dominates at higher concentrations of matrix polymer. The thermodynamic effect, manifested by a decrease in the apparent second virial coefficient in the total intensity light-scattering experiment and a decrease in kd(app) in the dynamic light-scattering experiment, appears to be the most significant effect of the matrix polymer in dilute solution.