Since trihalomethanes (THMs) and other volatile organic compounds (VOCs) were detected and measured in drinking water supplies in 1974, because of the frequent occurrence of these compounds and the potential health hazard they pose, several methods for detecting VOCs have been developed. The most widely accepted method for the analysis of THMs and other VOCs is a purgeand-trap method. In the analysis of VOCs by purge-and-trap,there are several factors which may give rise to errors. Some of the factors to be considered are purge time, carryover effect, cryofocusing temperature, and trap desorption temperature. In this study,many aspects of purge-and-trap were investigated. Understanding the sources of error makes it possible to adapt the analysis parameters to compensate for such effects.