Due to its reactivity, the OH radical is considered to represent the oxidative potential of the troposphere and by a simple photochem. box model, a steady state OH concn. of 1.6 * 106 mol./cm3 for the midday continental troposphere in the absence of hydrocarbons is detd. The concn. of OH depends on a fast photochem. cycle initiated by O3 and water vapor and driven by CO and NO; thus, OH is predicted to be higher by a factor of .apprx.4 in polluted continental air as compared with the clean troposphere. Using CH4 as a model compd., OH in the continental atm. is essentially unaffected for ambient CH4 concns. of 1.5 ppm; for the clean troposphere, OH is decreased by .apprx.25% compared to the CH4-free case. As a consequence of this, a slight shift of the oxidative potential of the troposphere from unpolluted maritime areas to the more polluted continents parallel with the temporal increase of the CH4 concn. is predicted.