Investigations on two commercially available permeation tubes for preparation of test gas mixtures for calibration purposes in trace gas analysis have shown that, depending on the type of permeation membrane, variations of carrier gas pressure may strongly affect the outcoming test gas concentrations. At constant atmospheric pressure both devices work with concentration fluctuations of about 3 to 5%. With carrier gas overpressures up to 2000 mbar the behaviour of permeation tubes with a rigid PTFE-membrane remains almost unchanged whereas test gas generators with soft membranes, e.g. silicon rubber, give a very strong decrease of permeation rate down to 17% of the value at atmospheric pressure. Consequently, the selection of permeation devices demands high carefulness with regard to the ambient pressure at the site of application.