A new indicator is proposed to det. whether tropospheric O3 prodn. in a specific area is limited by volatile org. compds. (VOC) or NOx. This indicator, tVOCOH/tNOxOH, describes the ratio of OH- lifetime vs. losses by reaction with VOC and NOx. Whereas tNOxOH can be obtained by conventional measurements, the new pump and probe OH approach makes it possible to also obtain tVOCOH. Indicator values above a threshold value of 0.2 +- 50% are representative of NOx-satd. conditions, where an increase of NOx emissions causes lower O3 prodn. For values <0.01, O3 prodn. is very insensitive to changes of VOC emissions. The robustness of this indicator against several parameters (temp., humidity, photolysis, initial O3 concn.) was tested in a box model and compared to the robustness of other earlier proposed indicators. In contrast to earlier proposed indicators, this new one is not based on photochem. produced long-lived species, but describes the instantaneous regime of an air parcel. Three-dimensional modeling showed this indicator is quite successful for estg. the impact of increased or reduced emissions on O3 concns. for each location in the modeled area. This will make it a very helpful tool for developing O3 abatement strategies.