We use concurrent morning peak observations of carbon monoxide (CO) and nitrogen oxides (NOx) to evaluate mobile emissions estimates for CO and NOx at Bogotá (Colombia), Buenos Aires (Argentina), Santiago (Chile) and São Paulo (Brazil). In all cities, molar ratios of CO to NOx decrease over the last 10e15 years. These ratios are not captured by available inventories. Comparison among inventories suggests that major uncertainties are linked to inadequate emission factors for CO and inadequate activity data for NOx. These results, in combination with previous studies, suggest that current NOx emissions are overestimated by a factor of up to 3 in Santiago and São Paulo, and Buenos Aires shows a slight overestimate by 20%. In the case of Bogotá we suspect that the current CO emission inventory is overestimated. Available observations provide valuable information, as exemplified hereby, but more careful attention must be paid to calibration and continuity of the stations.