In accord with the Porter equation, linear relationship has been found between the apparent and actual network-chain concentrations of DCP-cured NBR reference elastomers containing 18–39% ACN and 0–70 phr N550 black while Porter's k factors and, therefore, the extent of the restriction of swelling by the filler vary somewhat with the diluent. Taking this variation into account, a self-consistent relationship has been obtained between data on swelling in MEK (24 h) and those on equilibrium swelling in some fuel and lubricant ingredients (2 to 4 weeks). By using this relationship and the modified Flory-Rehner equation, the equilibrium swelling in fuel and lubricant ingredients of NBR test vulcanizates can be estimated from determining their nitrile content and from an MEK immersion test if these vulcanizates are filled with only N550 black, and if the only variables in their compositions are nitrile content, state of peroxide cure, and carbon black loading. The development of a similar technique to predict the swelling in mixed solvents of unknown NBR vulcanizates would probably involve serious difficulties which would stem from the presence of plasticizers, antioxidants, and fillers other than N550 black as well as from the assessment of the polymer-solvent interaction factor.