Polysulfides increase yield from kraft pulping by selectively oxidizing the reducing end groups of the polysaccharides thereby protecting them from hydrolysis. The principal polysulfide homologue formed in white liquor after addition of elemental sulfur was found to be the S-2(2-) species. Analysis of both cooking liquor and wood chips during early cooking suggests that the polysulfide present in the chips degrades more slowly than the fraction dissolved in the bulk liquor. The alkalinity inside the chips is lower than that in the bulk liquor. We propose that the rate of degradation of the polysulfides contained within the chip is correspondingly reduced. As pulping proceeds and the structure of the chips open up, the diffusion barrier across the chip is reduced and the polysulfide concentration in the chip and in the bulk liquor begins to even out. It follows that pre-equilibration of wood chips with polysulfide before pulping may reduce the optimum polysulfide dose.