The effects of dynamic vulcanization and blend ratios on mechanical properties and morphology of thermoplastic elastomeric (TPE) compositions, based on blends of nitrile rubber (NBR) and poly(styrene-co-acrylonitrile) (SAN), were studied. The TPE composition prepared by adding a rubber-curatives masterbatch to softened SAN yields higher mechanical properties than that prepared by adding curatives to the softened plastic-rubber preblend. The blends having a higher rubber-plastic ratio (60:40 to 80:20) display thermoplastic elastomeric behavior, whereas those having a higher plastic-rubber ratio (50:50 to 90:10) display the behavior of impact-resistant plastics. DSC studies revealed that NBR and SAN are thermodynamically immiscible. SEM studies of the thermoplastic elastomeric compositions show that SAN forms the matrix in which fine particles of NBR form the dispersed phase. It was further confirmed by dynamic mechanical thermal analysis. Dynamic vulcanization causes a decrease in the size of dispersed particles and improvement in mechanical properties. (C) 2003 Wiley Periodicals, Inc.