The effect of SO42- availability on the microbially mediated reductive transformation of As(V)-coprecipitated schwertmannite (Fe8O8(OH)3.2(SO4) 2.4(AsO4)0.004) was examined in long-term (up to 400 days) incubation experiments. Iron EXAFS spectroscopy showed siderite (FeCO3) and mackinawite (FeS) were the dominant secondary Fe(II) minerals produced via reductive schwertmannite transformation. In addition, ∼25% to ∼65% of the initial schwertmannite was also transformed relatively rapidly to goethite (αFeOOH), with the extent of this transformation being dependent on SO42- concentrations. More specifically, the presence of high SO42- concentrations acted to stabilize schwertmannite, retarding its transformation to goethite and allowing its partial persistence over the 400 day experiment duration. Elevated SO42- also decreased the extent of dissimilatory reduction of Fe(III) and As(V), instead favoring dissimilatory SO42- reduction. In contrast, where SO4 2- was less available, there was near-complete reduction of schwertmannite- and goethite-derived Fe(III) as well as solid-phase As(V). As a result, under low SO42- conditions, almost no Fe(III) or As(V) remained toward the end of the experiment and arsenic solid-phase partitioning was controlled mainly by sorptive interactions between As(III) and mackinawite. These As(III)-mackinawite interactions led to the formation of an orpiment (As2S3)-like species. Interestingly, this orpiment-like arsenic species did not form under SO4 2--rich conditions, despite the prevalence of dissimilatory SO 42- reduction. The absence of an arsenic sulfide species under SO42--rich conditions appears to have been a consequence of schwertmannite persistence, combined with the preferential retention of arsenic oxyanions by schwertmannite. The results highlight the critical role that SO42- availability can play in controlling solid-phase arsenic speciation, particularly arsenic-sulfur interactions, under reducing conditions in soils, sediments, and shallow groundwater systems.