Sulfidogenesis and iron reduction are ubiquitous processes that occur in a variety of anaerobic environments and profoundly impact the cycling of arsenic. Of the iron (hydr)oxides, ferrihydrite possesses one of the highest capacities to retain arsenic and is globally distributed within soils and sediments. Reactions occurring between sulfide, ferrihydrite and arsenic are partially governed by pore size heterogeneity existing in the subsurface. Pore domains mediate the distribution of redox gradients within soils and sediments; highly reducing environments often exist within soil aggregate micropores (less than 10 µM in diameter), and less reducing or oxic conditions within soil macropores (greater than 500 µM in diameter). Here we examine the behavior of arsenate (As(V)) and arsenite (As(III)) in column systems containing ferrihydrite coated sand and bacteria capable of iron, sulfur, and/or arsenate reduction. We have also developed a novel system for examining ferrihydrite transformations and arsenic mobilization across a diffusive gradient of sulfide, simulating sulfide diffusion in or out of a soil aggregate containing micropore domains. Initial column studies suggest that sulfide genesis mobilizes arsenic, possibly via the formation of soluble As-S species during the early stages of diagenesis. Progressive development into a sulfide dominated systems results in the repartitioning of arsenic into the solid phase. Diffusion studies reveal a complex set of iron (hydr)oxide-sulfide reactions occurring as a function of time, space, and sulfide diffusion rates into diffusion dominated pore domains containing ferrihydrite.