Tedlar bags are tested for their suitability for breath sampling for medical diagnostic purposes. Proton-transfer reaction-mass spectrometry was used to monitor the changes in composition of various mixtures contained in custom-made black-layered Tedlar bags. Characteristic ions at m/z 88 and 95 amu reflect considerable pollution from the bag material. The pollutant found on m/z 88 amu is most probably N,N-dimethylacetamide, a latent solvent used in the production of Tedlar film. Gas composition losses during filling were found to range from 5 to 47%, depending on the compound. Once stored, the half-lives of methanol, acetaldehyde, acetone, isoprene, benzene, toluene and styrene were estimated between 5 and 13 days. Losses from breath samples (52 h after filling) were found to be less than 10%. No observable decrease was found for ethylene over 3 days, using laser-based photoacoustic detection. For the use of Tedlar bags, a standardized protocol is advised, where the time point of analysis is fixed for all samples and should be kept as close as possible to the time of sampling.