Many coastal marine sediments display highly heterogeneous biogeochemistry due to complex biological and chemical interactions. Existing measurement techniques are limited in their ability to characterize the distributions of reduced species at high resolution in two dimensions. To obtain more detailed information than existing methods, a novel technique for the simultaneous high-resolution (1 mm), two-dimensional determination of porewater iron (II), and sulfide using a gel-based diffusive sampler was developed. A diffusive equilibration in a thin-film (DET) hydrogel was colorized by Ferrozine reagent, imaged electronically, and analyzed using computer imaging densitometry. With the selected gel parameters, the method detection limit for iron (II) was 0.6 μmol L-1, with an upper calibration limit of 500 μmol L-1 and relative standard deviations below 10% across the calibration range. This method was integrated into the existing diffusive gradient in a thin film (DGT) method for sulfide determination. Field deployments of a prototype probe, which measured an area of 170 × 80 mm, in estuarine sediment revealed complex, fine-scale, interlocking zones of iron (II) and sulfide. This confirms the need to assess more than a single dimension, at appropriately high resolution, to obtain an accurate representation of sediment geochemistry. Advantages of the technique include its high spatial resolution, minimal sample handling, flexible probe size, simultaneous measurement of two analytes at exactly the same location in the sediment, and the rapid production of data without requiring expensive analytical instrumentation.