Two types of particulate organic materials, pure cellulose and alder catkins from common alder (Alnus glutinosa), were used to determine and compare decomposition rates of POM in two zones of a small lowland stream – the surface water and the hyporheic sediments. Generally, decomposition rates of both POM types incubated within the hyporheic sediments were relatively very slow (k = 0.002–0.005 d–1) compared to that in the surface water (k = 0.02 d–1). Breakdown of both POM types was clearly influenced by water temperature. Invertebrates found on samples of both cellulose and alder catkins, incubated within the hyporheic zone, had much lower species richness compared to samples from the surface water. In the surface water, the dominant species on both materials was the amphipod Rivulogammarus roeselii. Hyporheic samples were dominated by chironomid larvae and tubificids.