By use of the data on the statistical sorption on solvent vapor, the Gibbs energies of mixing of cellulose nitrate (12.4-12.8% N) and nitrile-butadiene rubbers with various contents of acrylonitrile groups were calculated. It was shown that increasing fraction of polar groups in the rubber extended the range of compatibility of these polymers, until at 40% acrylonitrile the components became completely compatible.