The importance of biodiversity for the functioning of ecosystems is still very unclear. Many hypotheses, mainly based on terrestrial studies, have been formulated, focussing on the plant diversity-productivity relationship. In this study, diversity-related and species-specific effects of bacterivorous nematodes on the decomposition rate of cordgrass detritus Spartina anglica and on the associated microbial community were investigated using laboratory microcosm experiments. Four bacterivorous nematode species (Diplolaimelloides meyli, Diplolaimelloides oschei, Diplolaimella dievengatensis and Panagrolaimus paetzoldi) were added either separately or in combinations of 2 or 3 species to the microcosms. In contrast with previous reports, no stimulation of the decomposition process was observed in the presence of nematodes. Still, clear differences in process rates were found between nematode treatments. P. paetzoldi, reaching considerably higher densities than the other nematode species, suppressed bacterial activity and diversity, probably due to overgrazing. This was, however, not translated into slower decomposition. Within treatments exclusively containing monhysterid nematode species (D. meyli, D. oschei and D. dievengatensis), differences in microbial activity and decomposition rates were found, but again no link was observed between activity and decomposition. Our data did not support any hypothesis predicting enhancement of process rates with an (initial) increase in numbers of nematode species, or redundancy among the studied species. Rather, we obtained support for an idiosyncratic diversity model, as differences in the effect of bacterivore nematode species and species combinations could not be predicted at the start of the experiment. This could be explained by the inhibitory interactions between nematode species.