In this study, hydrogenated acrylonitrile butadiene rubber (HNBR, ZETPOL-2010L) and nitrile butadiene rubber (NBR, NIPOL-DN4555) were blended at different ratios in a Haake melt blender at 130degreesC. The HNBR and the NBR were of very similar acrylonitrile content and Mooney viscosity. The melt miscibility and solid-state properties were investigated by rheological, thermal, and mechanical testing and scanning electron microscopy (SEM) techniques. The dynamic viscosity of the blends followed the log-additivity rule, while the flow activation energy closely followed the inverse additivity rule. On the other hand, the storage modulus showed synergistic effects at all compositions, suggesting the presence of emulsion morphology at both ends of the composition range. For the 50/50 HNBR/NBR blend, the SEM micrographs suggest a uniform elongated structure. The thermal analysis showed the presence of two glass transitions, representing the pure components, at all blend ratios. suggesting the absence of segmental miscibility of the blends. The small-strain mechanical properties such as tensile modulus and yield stress followed linear additivity. However, HNBR and HNBR-rich blends were observed to strain harden at a rate higher than that of NBR. Induced crystallization of HNBR was suggested to be the reason for the strain hardening. The different rheological, thermal, and mechanical testing techniques agree in suggesting that the structurally similar HNBR and NBR are not thermodynamically miscible but mechanically compatible. (C) 2004 Society of Plastics Engineers.