The ammonia loss through NalophanTM bags has been studied. Ammonia was chosen as target compound in order to be representative of odorous molecules of small dimensions. The losses observed for storage conditions and times as allowed by the reference standard for dynamic olfactometry (EN 13725:2003) indicate that odour concentration values due to the presence of small molecules may be significantly underestimated if samples are not analysed immediately after sampling. The diffusion coefficient of ammonia through the NalophanTM film was evaluated using the Fick's law, and it turned out to be equal to 2.38E−12 (m 2 /s). The results and their theoretical interpretation indicate that concentration losses due to ammonia diffusion through the NalophanTM film can be decreased by using large bags and filling them up to their maximum capacity.