Cure kinetics, morphology and gas permeability constants of CIIR/NR, BIIR/NR blends were examined as functions of elastomer viscosities. Scorch time and mixing extent decrease on increasing the viscosity of the components; scorch time is also negatively affected by the presence of BIIR in the blend. Values of gas permeability constant of the blends do not linearly depend on the composition and are affected by the viscosity of the components, which determines the morphology and the carbon black distribution in the blend.