Abstract: Forensic laboratories around the world have seen an increase in the number of drug seizures containing methorphan. Levomethorphan is a narcotic and a controlled substance, and its enantiomer dextromethorphan is an antitussive agent used in over-the-counter medications. For the forensic analysis of seized drugs containing methorphan, it is important to report the stereochemical composition of the drug. Ideally, a method based on common forensic laboratory instrumentation is desirable. The use of the chiral derivatizing agent (−)-menthyl chloroformate followed by routine gas chromatography–mass spectrometry analysis of the derivative was shown to successfully determine the stereochemical composition of methorphan. This approach was applied to a street seizure containing methorphan proving that it was pure dextromethorphan. The derivatives of dextro- and levomethorphan were subjected to mass spectroscopic and nuclear magnetic resonance analysis, which confirmed that the structure of the derivatives remained unchanged as a result of the derivatization process.