The Community Multiscale Air Quality (CMAQ) model was used to predict O3-NOx-VOC chemistry for the Pacific Northwest and these results were evaluated by comparing to aircraft measurements of CO, NOy, O3, and VOCs collected during the Pacific Northwest field experiment in the summer of 2001 (PNW2001). The evaluation focused on three areas: 1) photochemical indicator values (O3/NOy), 2) accuracy of the emission inventory, and 3) VOC reactivity. The evaluation was performed for two modeling scenarios: a standard scenario and a reduced VOC scenario, which was developed based on the comparisons of measurements with the emission inventory. Results showed that model-predicted O3/NOy ratios were closely related to VOC-NOx sensitive conditions, with transitional values similar to those identified from previous studies. Peak O3 was associated with VOC-sensitive conditions, but these were not far from the transitional regime. The standard modeling scenario over-predicted peak O3 and the O3/NOy slope, indicating an overestimation of sensitivity to NOx, probably due to too much VOC in the emission inventory. The reduced VOC scenario resulted in better agreement with measurements in terms of peak O3 as well as O3/NOy correlations. Comparisons of observed CO and VOC to NOy ratios from the morning urban samples with those from the emission inventory also supported an overestimation of VOC in the standard scenario, with CO concentrations over-predicted by about 80% and the total VOC reactivity over-predicted by 30%. The standard modeling scenario substantially overestimated the reactivity from CO. The reduced VOC scenario showed generally good agreement with observations of the relative contributions to the total VOC reactivity.