The experimental design and the results of an interlaboratory study of U.S. Environmental Protection Agency (USEPA) method 501.1 to detect trihalomethanes in drinking water are described herein. In the method, trihalomethanes are extracted by an inert gas which is bubbled through the aqueous sample. The vapors are then trapped on a short column containing a suitable sorbent. The trapped compounds are subsequently thermally desorbed onto the head of a gas chromatographic column. An electrolytic conductivity detector is used to measure the compounds. 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. This report was submitted in partial fulfillment of contract 68-03-2856 by Monsanto Research Corporation under the sponsorship of the U.S. Environmental Protection Agency. The report covers a period from September 1979 to December 1982.