The technique of diffusive equilibration in thin-films (DET) was used to measure pore-water concentrations of dissolved Fe at high (1 mm) and ultra-high (400 μm) spatial resolution in the surface sediments (0–12 mm) and immediate overlying water of Esthwaite Water, UK. DET measurements were made using an unconstrained DET probe and a new constrained DET probe with 200-μm wide compartments in a 400-μm thick ceramic sheet. Good agreement between profiles obtained by conventional and constrained DET measurements confirmed the performance of the new procedure, which in principle can be used to measure most major components of pore waters. Single point maxima observed with conventional DET were revealed as systematic peaks with constrained DET. All Fe pore-water profiles showed tightly defined maxima in the surface sediments. Double maxima observed for both spring and winter were probably due to tightly defined zones of bacterial oxidation of Fe(II) although sampling artifacts could not be completely ruled out. Differences in ultra-high resolution vertical concentration profiles measured only 2 mm apart provided evidence of small scale horizontal heterogeneity. Such observations suggest that fluxes through the sediment water interface calculated from concentration profiles should be treated as spatially specific values. Average fluxes can only be obtained from several replicate measurements.