Composting of the organic fraction of municipal solid waste (OFMSW) reduces the amount of biodegradable waste landfilled. However, the final product or compost used as organic soil amendment shows a large presence of improper materials and alarming concentrations of heavy metals. In this work, 30 samples of OFMSW before and after composting have been characterized to determine qualitatively and quantitatively this contamination and its origin. In addition, technical features of the equipment installed in 10 waste treatment plants have been assessed because of their influence on the streams involved in the composting process. Results show 78.2% of the samples stabilized by composting to be organic matter and the rest corresponds to improper materials, mainly paper, plastic and glass. Origin is due to the composting feedstocks, the OFMSW obtained by size separation in trommels which, due to non-source separation and poor selectivity, contains one third of impurities. In seven of the 30 samples household batteries were found. (C) 2010 Elsevier Ltd. All rights reserved.