The relative apportionment of hydrocarbons (HCs) coming from mobile, fixed, and other sources (not correlated either to carbon monoxide [CO] or sul-fur dioxide [SO 2 ] emissions as solvent evaporation and biogenic sources) is calculated as previously proposed by Riveros et al. 1 through the linear ap-proximation [HC] total = [HC] 0 + m 1 [CO] + m 2 [SO 2 ], where m 1 and m 2 are fitted constants. The obtained apportionment with air samples taken in 1993 is con-sistent with the earlier published apportionment with air samples taken in 1992, validating the previous proce-dure. This analysis suggests that 75% of HC originate IMPLICATIONS The simultaneous measurement of HC and NO 2 in Mexico City's urban atmosphere was used to estimate the contri-bution of these two precursors to tropospheric O 3 forma-tion. This paper shows that HC and NO 2 measurements at noon (12:00–3:00 p.m.) are better predictors of maximal O 3 concentration than measurements at the concentra-tion peak (6:00–9:00 a.m.). This can be explained by the variable pollutant dilution associated with changes in at-mospheric conditions through the day. The approach used in this work to estimate HC and NO 2 contribution to O 3 formation gives useful information to evaluate antipollu-tion policies. from mobile sources, 5–18% from fixed sources, and 7—20% from other sources (mainly solvents and bio-genic sources). A similar analysis was employed to estimate the rela-tive contribution of HCs and nitric oxides (NO 2) to ozone (O 3) formation, the most important air pollutant in Mexico City. In this way, through a local lineation of O 3 isopleths, simultaneous measurements of HC and NO 2 in the atmo-sphere were fitted to the equation—[O 3 ] peak = [O 3 ] 0 + m a [HC] + m b [NO 2 ]—to predict O 3 peak. With these data, the ad-justed parameters show that NO 2 , not HC as was proposed previously, is the most important contributor to O3 formation.