Abstract: An equation correlating bound rubber content A and molecular weight M of the monodisperse polymer has been proposed: A = Ke-b/M(2)(Fig. 3) Parameter b was shown to be constant for two types of cis-1,4-polybutadi-ene rubber(Ni-and rare-earth-catalyzed). From the second derivative of equation(2), it can be shown that when the molecular weight of the polymer is smaller than b/2, bound rubber content will decrease strikingly. Starting from Equation(2), Equations(5), (6)and(7)have been derived, for calculating, respectively, the molecular weight distribution Ws(M) and the weight and number average molecular weights, Mw and Mn, s, of the soluble rubber in the carbon black compound. The only parameter is b. Calculated values from these equations are in agreement with experimental values from Gel Pe-rmention Chromatography.