Spectrophotometric and chromatographic studies on Spicer's high iron diamine reaction suggest that new red and blue oxidation products are formed, which become rapidly aggregated mainly to ferrous ions. Model experiments with pure substrates of known physical and chemical nature at pH 1.3-1.5 indicate that this reaction selectively demonstrates compounds carrying ester sulfate groups. Substrates with the highest charge density (e.g., heparin and dextran sulfates) give dark purple or violet colors, whereas others with lower charge density give brownish reddish or purplish shades.