Longitudinal ultrasonic velocity, and ultrasonic attenuation, were measured in Adiprene CM-NBR, Adiprene CM-NR, Adiprene E-NR, Adiprene E-NBR blend systems at room temperature, using pulse echo technique. The densities of the blend samples were measured by the immersion method. Glass transition temperatures were measured experimentally using ultrasonic and differential scanning calorimetry (DSC). Glass transition temperatures were also calculated theoretically, Results showed that the variation of both longitudinal ultrasonic velocity and density with composition is linear with Adiprene CM-NBR, Adiprene CM-NR, and Adiprene E-NBR blends indicating compatible systems. For Adiprene E-NR, the plots of ultrasonic velocity and density vs. composition deviate from linearity according to the incompatibility degree of this system. Results obtained from ultrasonic attenuation measurements showed that the excess variation of ultrasonic attenuation with composition in rubber blend samples is of relaxational type and only one maxima or minima appeared in case of compatible blends and two or more maxima or minima appeared in case of incompatible blends. The presence of a single transition temperatures observed in DSC thermograms and ultrasonic attenuation-temperature relation is a decisive confirmation of the formation of one single phase and therefore the compatibility behaviour of this blend. On the other hand, the presence of two transition temperatures is taken as evidence of phase separation and consequently the formation of two phases with incompatible behaviour of the blend system. (C) 2002 Elsevier Science B.V. All rights reserved.