Abstract: The excitation of the target nitrocompound with ultraviolet radiation ress in photodissociation yielding vibrationally excited NO with significant population of the v"=1 and v"=2 levels of the ground electronic state. As the population distribution of ambient NO favors the v"=0 level, discrimination between vibrationally excited NO and ambient NO is possible by probing the NO A-X (0,0),(1,1), and (2,2) bands near 226, 224, and 222 nm, respectively, employing (1+1) resonance-enhanced multiphoton ionization (REMPI). Many complex nitrocompounds cannot be photolyzed near 452 nm since their absorption cross sections are relatively small. Thus, the visible laser radiation is used to facilitate the detection of ambient NO and NO from NO.sub.2 by (2+2) REMPI and to discriminate these species from more complex nitrocompound analytes. The analytical utility of the present invention has been demonstrated at several photolysis/ionization wavelengths for NO/CH.sub.3 NO.sub.2 and NO.sub.2 /CH.sub.3 NO.sub.2 mixtures.