This paper discusses a novel application of PIGE for the determination of13C in breath. Samples of human breath, urea, glucose, benzamide, barium carbonate were analyzed against cylinder CO2 and graphite standard. An accuracy check of the13C determination (with reference to mass spectrometric “True” results) gave a relative error of only −0.4% for PIGE. The performance of different standards in this determination was assessed. Relative standard deviation for the determination of13C isotopic abundance in breath samples were <20%. Then, if a 25% change is conservatively assumed observable in13C abundance, an increase in13C percent isotopic abundance from the natural 1.11% (average) to only 1.39% may be detected.