A corrected equation for the concentrations in the drift tube of a proton transfer reaction-mass spectrometer (PTR-MS) is presented which accounts for the different mobility of primary ions and protonated analyte previous termvolatilenext term organic compounds (VOCs). As a consequence, the calculation of VOC concentrations from the measured count rates requires as well a correction factor, equalling the ratio of (i) the mobility of the protonated analyte VOCs to (ii) the mobility of the gases used to determine the transmission efficiency. However, such data do essentially not exist for the relevant VOCs. Published mobility data for small inorganic ions suggest that the correction can be larger than 20% and hence be a significant contribution to the overall accuracy of calculated concentrations. The correction emphasises the need to calibrate the PTR-MS in order to determine VOC concentrations accurately.