The influence of the size of plant litter particles on substrate induced respiration (SIR), inorganic N, respiration activity, protozoa and nematodes in soil was analysed. Finely ground (<2 mm sieve) and larger pieces (4×5 mm) of maize leaves (Zea mays L.) (C toN=20) and barley (Hordeum vulgare L.) straw (C to N=190) were added to an arable sandy loam, resulting in four treatments plus a control (no amendment). The microcosms were incubated at 10°C and sampled on days 0, 10, 16, 24 and 71. After day 10, nitrate concentrations in soils with barley and maize were <0.8 and >8.0 µg N g^-1 soil, respectively. More N was re-mineralised in the soils amended with finely-ground maize than in those with the coarse maize or barley straw. Respiratory activity peaked during the first few weeks when soils with maize had a higher respiration than soils with barley. During this period, respiration was higher in soils amended with the large pieces on nine out of 10 occasions. Microbial biomass measured as SIR was significantly higher in soils with maize than in those amended with barley, but no effect of particle size was observed (three-way ANOVA, P<0.05). Protozoan numbers were not affected by type of plant material, but significantly higher numbers were found in soil with finely-ground maize than in soil with large pieces (two-way ANOVA, P<0.05). In contrast, large pieces of maize or barley resulted in significant higher nematode numbers than ground material, and maize supported higher populations than barley (three-way ANOVA, P<0.05). The different response of protozoa and nematodes to particle size can be related to their life strategies: protozoa are numerous and have restricted mobility whereas nematodes are larger, with more mobility towards resources.