The effect of benzoic and stearic acids on 2-bisbenzothiazole-2,2'-disulfide (MBTS)- and tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of polyisoprene compounds, containing ZnO, was studied at 150 degrees C. In the presence of ZnO, the acids are rapidly converted to their zinc salts that, unlike the acids, do not attack accelerator polysulfides and, instead, promote vulcanization. The addition of accelerator polysulfides to the chain is not affected by zinc salts, but the rate of crosslinking of pendent groups is facilitated. In MBTS vulcanization, the promotion of crosslinking reactions reduces the number of pendent groups involved in cyclization reactions and higher crosslink densities are attained. In TMTD compounds, crosslink densities are increased on addition of ZnO and of ZnO/carboxylic acids. A mechanism is suggested to account for the promotion of crosslinking by the zinc salts. Bis(2-mercaptobenzothiazolo)zinc(II) and bis(dimethyldithiocarbamato)zinc(II) can act similarly to zinc benzoate or stearate to promote the crosslinking of pendent groups, but are less efficient. With benzoic acid, a limited attack by zinc benzoate on TMTP leads to a slightly lower increase in crosslink density than is achieved with the addition of ZnO alone.