A pollutant solid material known as "alperujo", which is the main by-product of the Spanish olive oil extraction industry, was composted with either grape stalk (GS) or olive leaf (OL) as bulking agents. The experiment consisted of two piles which were composted for 34 weeks using forced ventilation (Rutgers strategy) assisted by mechanical turning. Then, substrates were allowed to mature during 16 weeks. Temperature, pH, total organic matter, total nitrogen, C/N ratio, water-soluble organic carbon, carbohydrates and phenols, fats, as well as the phytoxicity of the substrates were evaluated in order to monitor the composting progress. The composting performance was influenced by the characteristics of the bulking agents added, GS contributing more than OL to improve the advance of the process. However, both mature composts were equally free of phytotoxicity and showed a similar composition. (C) 2005 Elsevier Ltd. All rights reserved.