The transient morphological development of blends of sytrene-butadiene rubber (SBR) and cis-polybutadiene (BR) is investigated as a function of the viscosity of the individual rubbers and also the energy input (using a power integrator) during the mixing process. The gum rubber blends consist of SBR:BR = 80:20 and 20:80 by weight ratio. The ebonite technique is used to prepare specimens for electron microscopic (EM) morphological study. The EM results reveal that these 80:20 and 20:80 weight ratio blends always consist of two phases. The minor component always forms the dispersed phase throughout the major component matrix. The viscosity of each blend, as well as the energy input during the mixing operation, strongly governs the resultant blend morphology. In the initial mixing stages, the domain sizes of the dispersed phase vary over a broad range. As more energy is introduced to the system, the range of domain