The determination of chlorophenoxyacetic acid herbicide residues was done with GC-ECD as pentafluorobenzyl derivatives. The herbicides were separated on a 3% Silar 10 CP column, which permits better resolution from interfering organic acids of environmental origin. The fortification study provides an 84–91% recovery for the water and sediment samples. The detection limits were established as 0.1 μg/1 for the water and 1.0–1.5 ng/g for the sediment samples. The use of a 3%Silar 10 CP packed column in the monitoring study as compared with a 1.5% OV-17 + 1.95% QF-1 one, shows the very small amount of contamination of the samples with these herbicide classes.