Summary Samples of the lichen, Cladina rangiferina , were exposed to various concentrations of aqueous SO 4 for 10 min, and 0.5, 1, 3, 6 and 12 h. K + ‐efflux curves (K + release versus ppm SO 2 (wt/wt, aqueous)) were constructed for each exposure period. In separate experiments, the photosynthetic 14 C fixation rates were measured after the SO 2 exposures. Plots of % total 14 C fixation v. ppm SO 2 (wt/wt, aqueous) were found to be linear for the 3 h and 6 h exposure periods, while plots of log (% total 14 C fixation) v. ppm SO 2 (wt/wt, aqueous) were linear for shorter exposures (10 min, 0.5 and 1 h). Threshold SO 2 concentrations for each exposure time were evaluated by extrapolating the K + ‐efflux curves, or their linear transformations, to zero K + release. Similarly, the threshold SO 2 concentrations that did not cause a reduction in 14 C fixation (relative to control samples not exposed to SO 2 ) were determined by extrapolation of the linear relationships mentioned. The threshold values obtained from the K + ‐efflux data were in good agreement with those evaluated from fixation rates. Aqueous threshold SO 2 concentrations were converted to gaseous threshold values by an experimentally verified relationship. The resulting gaseous SO 2 threshold concentrations, (SO 2 )g** in units of ppm(v/v) in air, were related to the exposure time in hours, t, through the following rectangular hyperbolic function: (SO 2 )g**=1.5 t −0.62 . Notwithstanding the uncertainties involved, extrapolation of this expression to exposure times considerably longer than the experimental range yielded threshold values for 6 months and 1 year in good agreement with mean SO 2 levels known from mapping studies to affect detrimentally sensitive lichen species.