The contribution of particle-size fractions to carbon (C) and nitrogen (N) mineralization of sludge compost was investigated. Particle-size fractionation was performed using "dry" (sieving of total dry compost) and "wet" (dispersion of compost in water, followed by sieving) fractionation methods, then C and N mineralization of the separated fractions were measured during incubation in soil. The "dry" fractionation did not allow the actual particle-size distribution of compost to be estimated accurately. Out of all the "wet" fractions, the [0-50 mu m] fraction was the most significant fraction in compost mass and contributed the most to the N mineralization of sludge compost in soil. Its low degradability, positive N mineralization and similarities with sludge OM suggest that the most humified sludge organic matter was located in this fraction, which would probably contribute to C storage and N availability after compost application in soil. Other fractions (>200 mu m) were more readily biodegradable and induced N immobilization. (C) 2009 Elsevier Ltd. All rights reserved.