An investigation into the occurrence of disinfection by-products (DBPs) was conducted for a period of two years (2001-2002) on the drinking water of Athens, Greece. Samples from four water treatment plants (WTPs) and from the distribution system of Athens were collected monthly and analyzed for DBPs. The concentrations of DBPs were determined by the gas chromatography-mass spectrometry (GC-MS) method, which included sample preparation with liquid-liquid extraction (LLE) for volatile DBPs and an acidic method of esterification for haloacetic acids (HAAs). The DBPs studied included trihalomethanes (THMs), haloacetonitriles (HANs), haloketones (HAKs), chloral hydrate (CH), chloropicrin (CP) and nine HAAs. The results of the analyses show the presence of DBPs in all water samples collected after prechlorination. The most commonly encountered DBPs were chloroform (CM), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA) and monochloroacetic acid (MCAA), while other volatile DBPs were detected at low concentrations. The annual mean concentrations of CM in four WTPs ranged from 8.0 to 42.5 mu g/l, while the annual mean concentrations of TCA, DCA and, MCA varied from 3.5 to 18.1 mu g/l, from 2.3 to 24.5 mu g/l, and from 1.1 to 61.8 mu g/l, respectively. Higher concentrations of THMs and HAAs were measured in the summer and fall compared to winter. In all cases the concentrations of TTHMs were lower than the maximum level set by the European Community.