Among the phthalic acid esters (PAEs) targeted by the United States Environmental Protection Agency (USEPA) as priority pollutants, diethyl-hexyl phthalate (DEHP) is the major pollutant identified at high concentration level in lagooning sludge (LS), at about 28.67 mg/kg, and in activated sludge (AS), at about 6.26 mg/kg. Other phthalic acid esters, such as di-butyl phthalate (DBP) and di-methyl phthalate (DMP) show very low concentrations. During sludge composting, after the stabilization phase, the subsequent appearance of DEP and then DMP occurred indicating that microbial metabolism begins by alkyl side-chain degradation before aromatic ring-cleavage. The appearance and accumulation of PAEs with a short alkyl side-chain in the last stages of AS and LS composting is suggested originating from the degradation of phthalates with a much long side-chain. The DEHP showed a rate of biodegradation that to Rows a first-order kinetic model during composting of both AS and LS. The calculated DEHP half-lives are 45.4 days for LS and 28.9 days for AS. The better DEHP biodegradation rate (2.4 x 10(-2) day(-1)) have been observed in the case of AS composting compared to LS compost (1.53 x 10(-2) day(-1)). The mono-ethylhexyl phthalates MEHP has been shown to follow the same order of biodegradation as DEHP indicating that the same mechanism is followed (hydrolysis or dealkylation of each DEHP side-chain). Composting could be suggested as a detoxification process for the removal of PAEs (mainly DEHP) from sludges after a sufficient time of treatment to provide a safe end product. (c) 2004 Elsevier Ltd. All rights reserved.