Tracer diffusion processes were studied using dynamic light scattering for polystyrene (PS) probe molecules in semidilute solutions of a poyl(vinyl methyl ether) (PVME) matrix confined in a porous glass bead. The porous silica medium was equilibrated with the ternary solution containing PS, PVME, and the common solvent 2-fluorotoluene. Three-way refractive index-matching between the PVME, the solvent, and the silica facilitated the extraction of diffusion data from the light-scattering autocorrelation functions. Unlike the less miscible system of PS, poly(tetrahydrofuran), and 2-fluorotoluene reported earlier, the tracer diffusion coefficient in the porous medium decreased monotonically as the concentration of PVME increased in the exterior solution. The decrease in the tracer diffusion coefficient at high matrix concentrations was similar in form between the interior of the porous medium and the exterior solution for three different molecular weight fractions of PS. The results of the center of mass tracer diffusion in the pore channels were analyzed in terms of reptation and constraint release mechanisms. In the molecular weight range studied, the data support the latter mechanism.