Fine particulate organic matter (FPOM; particles < 1 mm diameter) is an important source of carbon and energy in many aquatic systems. Despite this importance, decomposition rates for FPOM have rarely been determined, largely because of methodological constraints. In this study we developed models relating organic matter decomposition to microbial extracellular enzyme activity, and used these models to estimate decomposition rates for FPOM in a cypress swamp in southeast Louisiana. Very fine FPOM (< 0.25 mm diameter) in litter bags decomposed faster than coarser (0.25 – 1 mm) particles, with mass loss rates of 0.27%/d and 0.18%/d for the two sizes, respectively. However, microbial enzyme activity associated with unconfined FPOM in sediments was higher than that within litterbags. Enzymatic models predict that this material is decomposing more rapidly, at rates 1.5x to 4x that of the material in litterbags. These findings suggest that litterbags substantially underestimate processing rates of fine organic particles, which can more accurately be determined by measurements of microbial activity. Furthermore, the differences in processing rates between the different sizes of FPOM suggest that the concept of FPOM as a functionally distinct type of material (i.e. all organic particles < 1 mm diameter) is inappropriate.