The experimental design and results of a validation study for an analytical method to detect trihalomethanes in water are described herein. This method consisted of a liquid/liquid extraction using pentane and subsequent direct analysis by gas chromatography using an electron capture detector. The six concentrations of spiking solutions contained chloroform, bromodichloromethane, chlorodibromomethane and bromoform. The two waters used in the study were distilled and drinking water, both supplied by the individual laboratories. Statistical analyses and conclusions in this report are based on analytical data obtained by twenty collaborating laboratories. The report covers a period from September 1979 to December 1982.