Tracer diffusion coefficients, D, for four linear polystyrenes (Mw= 6.5 X 104,1.79 x 105, 4.22 X 105, and 1.05 x 106) have been measured in solutions of poly(vinyl methyl ether)/o-fluorotoluene, as functions of matrix molecular weight (Pw= 1.4 X 105,6.3 X 105, and 1.3 X 106) and matrix concentration (0 < c < 0.30 g/mL), by dynamic light scattering. The data have been corrected for the concentration dependence of the local friction, using pulsed-field-gradient NMR measurements of solvent diffusivity. The behavior of D(M,P,c) is not consistent with the basic reptation-plus-scaling approach, in that a regime where D ~ Ar2P°c’1,76is not observed. Apparent M exponents vary between -0.56 at very low c and -2.3 at relatively high P and c. A regime where D is independent of P is not established over this range of variables. The c dependence approaches power law behavior at high concentrations, where D ~ c-3’3. All the data can be collapsed to a master curve when plotted as D/D0versus c/c’, where c’ is approximated as(cpscpvme)0.5-The data can be interpreted in terms of a gradual transition to diffusion by reptation with increasing concentration, as suggested by more recent theoretical approaches. At the same time, however, it is not possible to eliminate other proposed mechanisms, such as those invoking cooperative chain motion or intermolecular hydrodynamic interactions, solely on the basis of measurements of D for linear polymers. In an accompanying article, measurements of D for 3-arm and 12-arm stars in the same matrix solutions are presented, which lend substantial support to the basic correctness of the reptation picture.