In this paper, we describe and use a relatively new technique - dynamic light scattering from refractive index-matched ternary solutions-to study a quantity very closely related to the self-diffusion coefficient in binary systems. We refer to this quantity as the tracer diffusion coefficient. This tracer diffusion coefficient is expected to behave in much the same way as the self-diffusion coefficient, in terms of its concentration and molecular weight dependencies. In this study, we use two compatible polymers, polystyrene and poly(vinyl methyl ether), and a solvent, o-fluorotoluene, chosen specifically because its refractive index matches that of the poly(vinyl methyl ether). The technique is advantageous in that it allows the experimenter to vary independently the molecular weight of both the probe and "invisible" matrix polymers, their individual molecular topologies, and the overall polymer concentration with relative ease. No special chemical tagging is required, although it must be borne in mind that we are not measuring self-diffusion but the diffusion of a dissimilar tracer. Our experiments probe the diffusion of linear polystyrenes in matrices composed of linear poly(vinyl methyl ether)/o-fluorotoluene. Our results show a crossover from non-free draining (Zimm) to free draining (Rouse) hydrodynamic behavior of polystyrene as the concentration of the invisible poly(vinyl methyl ether) making up the matrix is increased.