Inverse gas chromatography technique has been used to study the thermodynamic compatibility of the industrially important elastomers polybutadiene (BR) and polybutadiene copolymerised with acrylonitrile (NBR). The NBR used in this study had nitrile contents of 18 and 34%. The ratio of BR/NBR in blends varied between 1 and 0.25 in both cases and retention volume of twelve probes was measured at 80 degrees C. The Flory-Huggins interaction parameter X(23)’, computed using a standard procedure, and also the interaction parameter B-23 showed that BR and NBR are incompatible in all compositions and that incompatibility increases with nitrile content.