Fine particulate organic matter (FPOM) plays a critical role in structuring and sustaining stream food webs by providing an essential resource for various organisms. Our goal was to elucidate FPOM dynamics by determining chemical properties, microbial respiration, and in situ decomposition rates of different FPOM fractions in relation to the parent coarse particulate organic matter (CPOM). FPOM (100–500 μm) of defined quality was produced by feeding 5 types of CPOM to shredding amphipods (Gammarus spp.): wood, filamentous green algae, and conditioned leaves of ash, alder, and oak. Feeding and defecation of Gammarus homogenized POM of the different origins in terms of proximate lignin and nutrient content. FPOM had higher lignin content (20.5–45.6%) than did parental CPOM (5.7–26.8%), whereas molar C:N decreased during the conversion of CPOM (12–109) to FPOM (10–34). Microbial respiration rates on leaf-derived FPOM were lower (0.13–0.45 mg O₂ g⁻¹ C h⁻¹) compared to rates measured for parent CPOM (0.37–0.80 mg O₂ g⁻¹ C h⁻¹). Furthermore, microbial decomposition over 2 mo in a stream was slower for leaf-derived FPOM (k < 0.0015/d) than for the parent CPOM (k = 0.0013–0.0049/d), and this pattern resulted in a positive correlation between rates of microbial respiration and decomposition. Overall, our data indicate that transformation of CPOM to FPOM has a homogenizing effect toward lower C quality, which, in turn, reduces microbial activity and decomposition rate.