This study numerically investigates reactive pollutants (NO, NO2, and O3) flow and dispersion in a street canyon with a canyon aspect ratio of one using a simple chemical reaction mechanism. A line source for medium emissions of NOx was considered in the presence of background O3 concentration. The contours of those species (NO, NO2, and O3) was produced and the photo-stationary state defect was calculated. It was found that the photo-stationary state defect was a sensitive indicator of reactive mixing within the canyon and the greater the defect, the greater the local state of chemical instability. High values were found near the roof level along the downwind wall and low values were found at the ground near the upwind wall. The results demonstrate that the anticorrosive materials should be positioned or construed at the downwind side of the canyon.