The pyrolysis gas chromatography mass spectrometry of Ibuprofen, Fenoprofen, Naproxen and Ketoprofen, a series of anti-inflammatory propionic acid derivatives, is shown to proceed via decarboxylation and elimination to yield characteristic ethyl and vinyl fragments. The pyrogram enables the identification of the drug to be achieved as the pure compound, in a formulated dosage form or excreted in urine. The presence of metabolites derived from Ibuprofen causes four new fragments to be observed in the urine pyrogram. The identification of 16 components of the control urine pyrogram is presented.