A new approach for rapidly analysing chlorophenoxy acid herbicides in water is presented. The chlorinated acids are derivatised with dimethyl sulphate in the water sample itself (800 μl) and, next, the methyl esters are extracted with 800 μl of n-hexane. A 200-μl volume of the extract is injected into the GC-MS system. The miniaturisation of both the methylation and extraction steps could be implemented because of the use of large-volume on-column injection and mass spectrometric detection. The optimisation of the methylation reaction for the simultaneous determination of (3,6-dichloro-2- methoxy)benzoic acid, (2-methyl-4-chlorophenoxy)- and (2,4- dichlorophenoxy)acetic acids, (±)-2-(4-chloro-2-methylphenoxy)- and 2-(2,4- dichlorophenoxy)propanoic acids and 4-(4-chloro-2-methylphenoxy)- and 4-(2,4- dichlorophenoxy)butyric acids showed that tetrabutylammonium salts act as catalysts. Addition of sodium hydroxide was required to obtain quantitative reaction yields for 4-(4-chloro-2-methylphenoxy)- and 4-(2,4- dichlorophenoxy)butyric acids. The methylation-cum-extraction procedure takes only 3 min per sample for a batch of seven samples. Linear calibration plots were obtained for the complete procedure and the limits of detection were of 10-60 ng/l with a signal-to-noise ratio (S/N) of 6. Relative standard deviations ranged from 8 to 15% (n = 7) for analyte concentrations of 0.5 μg/l in surface water. (C) 2000 Elsevier Science B.V.