An exptl. method has been developed to study the formation of OH radicals and of NO2 in the integrated oxidn. chain of hydrocarbons under NOx-rich tropospheric conditions. In this technique the oxidn. of hydrocarbons is initiated by laser pulse photolysis of Cl2 and the temporal behavior of OH and NO2 is monitored by direct time-resolved cw-laser absorption and LIF (laser-induced fluorescence), resp. Combined with computer modeling of the complete oxidn. mechanisms the results allow the detn. of (1) the extent of NO/NO2 conversion and hence the O3 formation potential of individual hydrocarbons and (2) the branching ratio of HO2 formation relative to other reaction channels of alkoxy radicals. First applications of this technique to the oxidn. of C2H6 and C3H8 are presented.