Previous geochemical and petrographic studies have only partly revealed the history and origins of solid bitumens from the East Papuan Basin, Papua New Guinea. In particular, the alteration phenomena, the distribution of hydrocarbon components and the increasing yields of pyritic sulfur indicate major effects of biodegradation and reduction of sulfate to sulfide. Sulfur isotope analysis of the three main sulfur fractions (elemental, sulfate and pyritic) showed these to cluster closely around a δ34S value near -25‰ An unlimited source of non-marine sulfate of constant isotopic composition is visualized. However, one clear trend in the data is the increase in the 34S content of pyrite as abundance increases. Such an isotopic relationship might well occur where, on reduction of a pool of sulfate, an early loss or reduction of 32S species would result in an increase in the 34S contents of the residual sulfate and product pyrite. Where the oxidation of pyrite to sulfate and sulfur occurs, the relatively greater 34S content of the elemental sulfur is consistent with previous data.