The conclusions of the present work may be summarized as follows. 1. The effective volume of carbon black in rubber, as measured by the low-strain equilibrium modulus, is equal to the true volume of carbon plus about half of the occluded volume of rubber calculated from the DBP absorption of the carbon black. 2. Breakdown of aggregates on incorporation in rubber may diminish the volume of occluded rubber; thus, calculations based on the same modulus data would give a somewhat higher effectiveness factor—perhaps of the order of 0.6, depending on the degree of breakdown assumed. 3. Under non-equilibrium conditions various processes may either raise or lower the measured modulus and (thus) the occluded volume effectiveness factor : (a) At moderate to high strains, strain amplification raises the modulus. (b) At low temperatures, amplification of the rate of strain raises the modulus (relative to that of the gum). (c) In a highly loaded vulcanizate, the secondary network of carbon black raises the modulus measured at low strain and low or moderate temperature. (d) The modulus can be lowered by molecular slippage, especially with thermal blacks, or with graphitized furnace blacks, at elevated temperatures and/or high strains.