3-Monochloro-1,2-propane diol is a suspected carcinogen found in hydrolysed vegetable protein products such as soy sauce. A method is described for the analysis of 3-monochloro-1,2-propane diol in soy sauce by gas chromatography-mass spectrometry at a concentration range of 1–5000 ng g−1 using 4-heptanone as the derivatizing ketone and 3-monochloro-1,2-propane diol-d5 as the internal standard. The limit of detection for the method in the soy sauce matrix was 0.48 ng g−1 and the limit of quantification was 1.2 ng g−1.