The water effect on peroxy radical measurement by chemical amplification was determined experimentally for HO2 and HO2+OH, respectively at room temperature (298±2) K and atmospheric pressure (1×105 Pa). No significant difference in water effect was observed with the type of radicals. A theoretical study of the reaction of HO2·H2O adduct with NO was performed using density functional theory at CCSD(T)/6-311 G(2d, 2p)//B3LYP/6-311 G(2d, 2p) level of theory. It was found that the primary reaction channel for the reaction is HO2·H2O+NO→HNO3+H2O (R4a). On the basis of the theoretical study, the rate constant for (R4a) was calculated using Polyrate Version 8.02 program. The fitted Arrenhnius equation for (R4a) is k = 5.49×107 T 1.03exp(?14798/T) between 200 and 2000 K. A chemical model incorporated with (R4a) was used to simulate the water effect. The water effect curve obtained by the model is in accordance with that of the experiment, suggesting that the water effect is probably caused mainly by (R4a). The water effect on peroxy radical measurement by chemical amplification was determined experimentally for HO2 and HO2 + OH, respectively at room temperature (298 ± 2) K and atmospheric pressure (1 × 105 Pa). No significant difference in water effect was observed with the type of radicals. A theoretical study of the reaction of HO2 · H2O adduct with NO was performed using density functional theory at CCSD (T) / 6-311 G (2d, 2p) // B3LYP / 6-311 G (2d, 2p It was found that the primary reaction channel for the reaction is HO2 · H2O + NO → HNO3 + H2O (R4a). On the basis of the theoretical study, the rate constant for (R4a) was calculated using Polyrate Version 8.02 program. The fitted Arrenhnius equation for (R4a) is k = 5.49 × 107 T 1.03exp (? 14798 / T) between 200 and 2000 K. A chemical model incorporated with (R4a) was used to simulate the water effect. The water effect curve obtained by the model is in accordance with that of the experiment, suggesting that the water effect is probably cau sed mainly by (R4a).