A profiling method for the identification of impurities found in seized 3,4-methylenedioxymethamphetamine (MDMA) tablets is presented. Impurities of interest are extracted from an alkaline solution (pH 12.8) by diethyl ether and submitted to gas chromatography (GC)-mass spectrometry (MS) analyses. Identification of impurities is performed by electron impact ionization (Ei) mass spectrometry and confirmation by positive chemical ionization (Ci+) MS or, when possible, MS/MS (MS(2)). Repeat extractions of the same sample give an average relative standard deviation (R.S.D.) of less than 8% within the same day and 15% between days (results were obtained after normalization by the sum of peak areas, each one being acquired by selected ion monitoring (SIM)). Possible application toward batch comparison of samples is discussed. Chromatographic profiles are compared using the cosine function for evaluating similarity and/or dissimilarity among exhibits.