In blends of rubber and low molecular weight resins, the compatibility of the system controls the viscoelastic properties and ultimately the performance of the composition as a pressure sensitive adhesive. The effect of the resin molecular weight on compatibility was examined by studying rubber-resin blends prepared from resins which represent a range of molecular weights. Viscoelastic properties were measured using a mechanical spectrometer on 1:l blends of rubber and a series of polystyrene resins and poly(vinylcyc1ohexane) resins. Based on plots of G' and tan 6 vs. temperature, blends of natural rubber and polystyrene resin show incompatibility at resin M,,, of about 600 and above. Blends of natural rubber and poly(viny1 cyclohexane) are incompatible at resin M, of about 1800, but are compatible at M , of about 650. Blends of styrene-butadiene rubber and polystyrene resins are compatible at resin M,,, of about 650 but appear to contain a low volume incompatible phase at M , of about 900. Therefore, the compatibility of a rubber-resin blend depends upon the molecular weight of the resin. Even systems expected to be compatible will show evidence of incompatibility as the molecular weight of the resin is raised above some limiting value.