Abstract Following our recent observation that Pseudomonas bacteria in vitro emit hydrogen cyanide, we have found it necessary to investigate the ion chemistry of this compound and to extend the kinetics database for selected ion flow tube mass spectrometry (SIFT‐MS) to allow the accurate quantification of HCN in moist air samples, including exhaled breath. Because of the proximity of the proton affinities of HCN and H 2 O molecules, the presence of water vapour can significantly distort HCN analysis in the presence of water vapour and a more sophisticated analytical procedure has to be developed. Thus, the reactions of H 3 O + (H 2 O) 0,1,2,3 ions with HCN molecules have been studied in the presence of varying concentrations of water vapour, reactions on which SIFT‐MS analysis of HCN relies. The results of these experiments have allowed an analytical procedure to be developed which has extended the kinetics database of SIFT‐MS, such that HCN can now be quantified in humid air and in exhaled breath. Copyright © 2004 John Wiley & Sons, Ltd.