Dynamic light scattering has been used to follow the tracer diffusion of polystyrene spheres (R approximate to 200 nm) in dilute, semidilute, and entangled solutions of poly(vinyl methyl ether) (M(w) = 1.3 x 10(6)). Over this range of matrix concentrations, 0 less than or equal to e[eta] less than or equal to 36, the diffusivity drops by almost 5 orders of magnitude. Near c(*) (approximate to[eta]-(1)) for the matrix, the diffusivity exceeds that estimated from the bulk solution viscosity via the Stokes-Einstein relation by a factor of about 3. Such "positive deviations" from Stokes-Einstein behavior have been reported previously in several systems. However, once the matrix concentration is sufficiently high for entanglements to be effective, Stokes-Einstein behavior is recovered. This new result was. confirmed via forced Rayleigh scattering. In-addition, these data can reconcile measurements of sphere diffusion with reptation-based models fdr chain mobility in well-entangled systems. The behavior near c(*) is discussed,is terms of the matrix correlation length, xi, which has a maximum at xi approximate to R(g) for c approximate to c(*). It is noted that the fluid; layer within a distance w of the sphere surface will, in general, differ in composition from the bulk solution, and consequently the sphere mobility may well not sense the macroscopic solution viscosity, particularly near c(*). As a corollary, for large matrix chains, dynamic light scattering may not monitor the long-time diffusion of the spheres near c(*), because q xi approximate to qR(g) x 1, rather than q xi << 1.