The technique of diffusive gradients in thin films (DGT) with measurements by computer-imaging densitometry (CID) was used to study the in situ, two-dimensional distribution of sulfide-producing microniches in sediments from a eutrophic lake (Esthwaite Water, UK). The DGT-CID technique precipitates and immobilizes the net pore-water flux of dissolved sulfide as black Ag2S by reaction with a AgI binding gel. The mass of accumulated sulfide is then determined from a scanned grayscale image of this gel. DGT probes deployed in laboratory mesocosms of homogeneously mixed sediment, then analyzed at high spatial resolution (~0.1 mm), showed that apparent niche areas (operationally defined by DGT-CID) were <1 mm2 for 30% of the niches. In eight DGT probes deployed in undisturbed sediment cores, the proportion of microniche-related sulfide flux was ≥1–8% of the total horizontal net pore-water flux of sulfide. This study suggests that microniches that introduce local redox gradients are common in sediments. As fluxes within these microenvironments can considerably exceed background values, consideration of the dynamics of the biogeochemical processes occurring at these sites is likely to be key to improving our understanding of early diagenesis. Measurement procedures and three-dimensional reaction-transport models should be designed with the aim of furthering understanding of the complexities associated with locally supplied particles of reactive organic matter.