The viscosities of miscible blends of 1,4-polyisoprene (M-n=78 kg / mol) /poly (vinylethylene) (M-n = 10 kg/mol and 120 kg/mol) and polystyrene (M-n=30 kg/mol)/poly(vinyl methyl ether) :M,, = 105 kg/mol) were measured as a function of temperature and composition. These results, along with literature data for poly (ethylene-alt-propylene)/h cad-to-head polypropylene [Gell (1996)] and molecular weight blends of 1,4-polybutadiene [Wang et al. (2003)], were compared to the predictions of a previously published model [Haley and Lodge (2004a)]. The agreement between theory and experiment is generally good, though some quantitative discrepancies are apparent. A new tube dilation model that predicts the terminal relaxation time of the slower moving component in a blend as a function of composition is presented. This model enables improved visosity predictions, and in several cases produces near quantitative predictions. (c) 2005 The Society of Rheology.