Some samples of soy sauce were examined for the presence of ethyl carbamate at a trace level. Solid-phase extraction with Celite and selected ion monitoring using a gas chromatograph-mass spectrometer were applied to analysis for ethyl carbamate in soy sauce. The method was simple and proved to be reliable for the analysis of soy sauce. Two soy sauce samples of 26 collected from Japanese and American markets had over 20 ppb of ethyl carbamate. Time course study of selected raw soy sauce samples that had been heated indicated that ethyl carbamate could be formed in the pasteurization process through ethanolysis of some precursors, as reported in wine, Japanese rice wine, or stone fruit brandy. In the case of soy sauce, citrulline would be the precursor according to examinations of a variety of experimentally fermented raw soy sauce samples. An accumulation of citrulline in raw soy sauce was concluded to be responsible for the occurrence of ethyl carbamate in soy sauce.