1. Analysis shows that, subject to certain limitations, the modulus of a loaded stock (M*) depends on the modulus of the rubber matrix (M), according to the equation: M*=M(1+2.5ϕ) where 100ϕ is the volume percentage of filler. When these limitations are fulfilled, the effect of compounding on modulus is, therefore, independent of the particle size of the filler. The assumptions on which this equation is founded are as follows: (1) the filler particles are spherical; (2) there is complete adhesion between rubber and filler; (3) the elongation is small; (4) the filler is completely dispersed; (5) the volume loading is small; (6) the filler particles are sufficiently large that the molecular structure of the rubber may be neglected. 2. The stresses about a filler particle have been derived mathematically. 3. Experimental data check the calculations for the following fillers: P-33, Thermax, and whiting. Catalpo clay presents some anomalies because of its acicular particles. 4. Carbon black does not conform to the calculations. This is attributed to the fact that it is strongly flocculated in rubber. 5. Zinc oxide (Kadox or XX zinc oxide), which should conform, because it is well dispersed in rubber, causes abnormally large increases in modulus, presumably because of alteration of the type of cure and consequent alteration of the modulus of the rubber matrix.