A study has been made of the mechanics of mixing of carbon black in rubber by determining the influence of agglomerates in the mix on the final properties of the vulcanizate. It is suggested that the mixing starts with the forming of soft agglomerates which contain polymer and black in about the proportion of the void ratio aa determined by oil absorption. In subsequent stages these agglomerates are torn apart— to smaller and smaller sizes and finally dispersed. When the agglomerates that are first formed in the mix are left intact as is the case in a very short mixing cycle, a noticeable deteriorating effect is found on the mechanical properties relating to rupture (tensile, abrasion). On the other hand, viscosities and moduli for short mixing cycles can be higher by a factor of 2 or more than the final viscosity and modulus after all the black is apparently well dispersed. This has been shown both experimentally and theoretically. Hard particles have also been added intentionally to assess the effect of their size, frequency and chemical nature. It appears that hard particles in an amount of 10–20% of the total black become very harmful at an average diameter of about 1 µ. At the 0.5 µ size, particles of medium thermal black are almost without influence on service properties of the vulcanizate such as tensile and tear strength, abrasion resistance, etc. Above the critical size, the size of the particle has little influence on the reduction in mechanical properties. This reduction is determined by the amount (by weight) of the coarse particles present. The effect of these hard particles on tensile and abrasion is much greater than that of the soft agglomerates formed in short mixing cycles.