Controlled‐ingredient‐distribution mixing technology has been exploited in a study of the covulcanization of an SBR‐NBR blend. The technique of differential solvent (or oil) swelling is applied to indicate the existence or absence of interfacial bonds between the SBR and NBR rubber phases. The analysis of swelling based on the fact that (1) NBR is hardly swollen in ASTM #2 oil, while SBR is highly swollen; and (2) the swelling restriction of the SBR‐rich compounds due to the presence of NBR is analogous to the swelling restriction of elastomer networks containing reinforcing fillers. The results on oil swelling, tensile properties, tear strength, and also flex crack growth resistance all indicate that interfacial bonds between the NBR and SBR phases can be achieved by appropriately controlling the distribution of black and other ingredients in the individual rubber phases.