Measurements of NO and NO2 are reported using a highly modified photofragmentation two-photon laser-induced fluorescence (PF-TP-LIF) instrument. Available evidence suggests that the changes made substantially reduced wall decomposition of labile NOy species. In sharp contrast to the results reported from NASA's 1991 PEM-West A program where the median value for the ratio (NO2)meas/(NO2)calc was 3.36, the PEM-Tropics A observations produced a value for this ratio of 0.93. This represents the first time that remote upper tropospheric NO-NO2 data have shown a high degree of correspondence with current photochemical mechanisms. ¿ 1999 American Geophysical Union