Surface water samples from 45 sampling points across Portugal have been analyzed monthly by automated Solid Phase Extraction (SPE) and Gas Chromatography-Mass Spectrometry (GC/MS) in SIM and SCAN modes. Sampling was carried out during a period of six months (April-September, 1999), in order to determine the levels of contamination of 72 organic compounds mainly selected from the EC Council Directive list (CEC 76/464/EEC). Atrazine, simazine, deethylatrazine, alachlor, metolachlor, Irgarol, terbuthylazine, tributhylphosphate, 2,4,6-trichlorophenol, 2-chlorophenol, 3-chlorophenol, pentachlorophenol, dimethoate, lindane, linuron, propanil, o,p'-DDD, o,p'-DDT, o,p'-DDE and malathion were the main detected compounds. Although Irgarol, terbuthylazine, deethylatrazine and deisopropylatrazine are not listed in the EC Directive, these herbicides and transformation products are currently in use and they were also included in the monitoring program and detected at some sites. Fifty-nine percent of the samples were below 0.1 microg l(-1), the detection limit required the EEC Drinking Water Directive (DWD). Some compounds were detected at values higher than the DWD, e.g. atrazine (2.7 microg l(-1)) and alachlor (5.43 microg l(-1)) in April. Simazine was also present with a high value (2.3 microg l(-1)) in September. Tributhylphosphate (5.40 microg l(-1)) and 2,4,6-trichlorophenol (2.31 microg l(-1)) were also found at high concentrations near industrial sites. Chemometric interpretation of the more representative contaminants was performed by means of Principal Component Analysis, allowing the resolution of the more relevant contamination sources and the description of their temporal and spatial (geographical) distributions.