Hydrolytic enzyme activity, surfactancy, and dissolved organic matter in the digestive lumens of 19 benthic echinoderm and polychaete species were examined, using consistent and quantifiable methods. Enzyme activities were compared with those of extracellular enzymes from ambient sediments. Enzyme activities ranged over five orders of magnitude, with averages decreasing in the order polychaetes > echinoderms > sediment. Highest activities in animals were usually associated with the fluid phase in midgut sections, with posteriorward decreases indicating little export to the external environment. At some phyletic levels, activity correlated inversely with animal size. Hydrolase patterns reflected food type; for example, high lipase:protease ratios in carnivores reflected esterified lipids in their diets. High surfactant activity was found in gut sections having high enzyme activity. Deposit feeders had the most intense surfactancy, including evidence for micelles. While enzymes reflected the biochemical nature of the digestible food substrate regardless of feeding mode (e.g., deposit vs. suspension feeder), surfactants reflected dilution of this digestible substrate with mineral grains. Dissolved organic matter levels were high, with amino acids reaching levels > 1M and lipids commonly 1 g L−1. Among polychaete deposit-feeders, low molecular weight amino acids reflected the composition of the food substrate, but were present at much higher concentrations than could be explained by sediment present in the gut—suggesting longer residence times for fluid than for transiting sediment particles. Deposit feeder digestive fluids are better able to solubilize sedimentary food substrates than are sedimentary extracellular enzymes, owing to either more powerful solubilizing agents or to their deployment in freely diffusing, dissolved form. Gut environments may lead to chemical condensation as well as solubilization reactions.