Blends of an ethylene/vinyl acetate copolymer (EVA) and polyisobutylene of various compositions were prepared by mechanical mixing at a temperature above the melting point of EVA (T-m(EVA)) but below the upper critical solution temperature of 170 degrees C for given blends. The rheological properties of the components and blends were studied in the region of small-amplitude oscillating deformation at temperatures above and below T-m(EVA) in the frequency range of 0.01-100 rad/s. At temperatures lower than T-m(EVA) the rheological properties were deter mined by the existence of the yield stress. With diminishing frequency, the viscosity increased, and the plateau in the relaxation spectrum at low frequencies broadened. The morphology of the blends depended on the conditions of sample heating. The introduction of a finely dispersed filler into the blends led to an anomalous drop in the viscosity. The morphology of the systems that arose by mechanical blending of the molten components was the important factor in the rheological behavior. The observed effects were examined in the framework of the concept of structural networks formed in melts by nonmelted crystallites of EVA. (c) 2006 Wiley Periodicals, Inc.