Abstract Poor dispersion of fillers in rubber compounds has been a troublesome problem since the beginning of the rubber industry. An example of this kind of difficulty is the presence of white pigment lumps in black or dark-colored compounds that are surface buffed, such as some types of shoe soling. Chemical analyses of filler agglomerates removed from a large number of unvulcanized and vulcanized rubber compounds revealed the fact that in practically every instance a substantial amount of zinc oxide was present. This is of major importance since good dispersion of zinc oxide is necessary for the proper vulcanization of a rubber compound. It is quite difficult to identify positively the specific variable or variables in any one given formulation which may cause filler agglomerates. They may be due to the composition of one or more of the ingredients present in the mix, the order in which the ingredients are added, or to the mixing procedure. However, a series of tests indicated one major cause, the addition of acidic materials such as fatty acids, organic acid vulcanization retarders, or acidic accelerators to natural rubber or some synthetic rubbers simultaneously with basic activators such as lime, litharge, magnesia, and zinc oxide, or with some basic fillers such as finely divided calcium and magnesium carbonates. High-temperature mixing (about 325° F) gave better dispersion than low-temperature mixing (about 220° F) when acidic or basic materials were added simultaneously, but in some instances poor dispersion resulted even at temperatures up to 325° F. This was particularly true of litharge and magnesia. It was also found that when zinc oxide and stearic acid were added to synthetic rubber at the same time, dispersion was better in some types of rubber than in others. The stiffer mixes of NBR (nitrile) and cold, unfilled SBR gave better dispersions than those of the softer IIR (butyl) and oil-extended SBR. Regardless of the type of rubber being used, the safest procedure to insure satisfactory dispersion of inorganic oxides is to add any acidic materials separately during the mixing procedure. This approach is particularly necessary if the mix contains resin acids and/or organic acid vulcanization retarders such as benzoic or salicylic acids. If it is not feasible to add the acidic and basic compounding materials separately to the rubber, then the inorganic oxides should be added to the mix in the form of masterbatches.