Nitric oxide concentrations in a portion of the exhaust of a diesel engine operated with equivalence ratios between 0.25 and 0.75 were reduced by up to 98 percent by the addition of cyanuric acid. The cyanuric acid was combined with the exhaust gas in an electrically heated quartz flow reactor. The effects of the key process parameters (temperature, exhaust gas composition and residence time, and the overall engine equivalence ratio) on NO reduction by cyanuric acid were investigated. Nitric oxide reduction was evident at flow reactor temperatures above 700 K. The maximum nitric oxide reduction varied from 80 percent for a reactor temperature of 1180 K and an engine equivalence ratio of 0.25 to 98 percent for a temperature of 1120 K and an equivalence ratio of 0.75. The temperature range over which 60 percent or greater nitric oxide reduction was obtained was 1100 to 1340 K. Increasing the exhaust gas carbon monoxide concentration lowered the required reactor temperature and increased the temperature range for significant nitric oxide reduction. Increasing the exhaust gas nitric oxide concentration lowered the ratio of cyanuric acid to nitric oxide required for maximum nitric oxide reduction.