Linear viscoelastic properties of linear and star-branched polymer solutions are investigated experimentally and theoretically. Two series of high molar mass 1,4-polyisoprene linear and three-arm star polymers blended with marginally unentangled linear chains of the same chemistry form the focus of this study. We find that, irrespective of polymer architecture, unentangled linear molecules dilate the entanglement environment in the blends in a manner consistent with expectations for a Θ-solvent. A tube model analysis used in an earlier study [Lee, J. H.; Fetters, L. J.; Archer, L. A. Macromolecules 2005, 38, 4484], for predicting linear viscoelastic properties of branched melts, is extended to describe relaxation dynamics of entangled star-branched polymer solutions. We find that, by simply rescaling the model parameters to account for Θ-solvent dilution effects, this model quantitatively describes relaxation dynamics in concentrated star and linear polymer solutions, without adjustable parameters. At lower polymer concentrations, however, the tube model systematically overestimates the effect of solvent dilution on the dynamic moduli. These observations are discussed in terms of the universality of the dilution exponent and breakdown of dynamic dilution at low degrees of arm entanglement.