CO and NOx emissions from motor vehicles are a major source of these pollutants in urban environments. Comparison of CO / NOx molar ratios in winter in Boise, Id to those predicted by the motor vehicle emission models MOBILE6.2 and MOVES are presented. MOVES is the new emission model developed by the US EPA to replace MOBILE. During the day the observed CO/NOx ratio was 4.6 compared to 13.2 and 9.1 predicted by MOBILE and MOVES. While MOVES predicts a significantly lower CO / NOx ratio than MOBILE it is still significantly greater than observed. Excellent agreement between observations and MOVES was obtained if off-network emissions were excluded from the model. This suggests that cold start emissions are over predicted in vehicle emission models.