Gaseous toxic air pollutants are normally associated with acute short-term and chronic long-term health effects, and hence measurement of these pollutants is essential on a daily basis to understand the science of their occurrences. In this study, gaseous O3, SO2, NO2, NH3, HNO2, HNO3, and HCl and water-soluble inorganic ions (Na+, K+, Ca2+, Mg2+, NH4 +, NO3 -, SO4 2-, and Cl-) in PM2.5 were measured during a summer in Kanpur, India, using an annular denuder technique on a time resolution of 12 h. The denuder technique revealed that evaporation losses of NH4 +, Cl-, and NO3 - from a Teflon filter and trapped in nylon filters were temperature sensitive. The evaporation losses were 17.8 ± 8.7%, 29.5 ± 19.3%, and 37.1 ± 25.9% during daytime, and 10.2 ± 6.4%, 17.7 ± 10.1%, and 19.3 ± 12.2% during nighttime for NH4 +, Cl-, and NO3 -, respectively. Overall, 24 h average concentrations of O3, SO2, NO2, NH3, HNO2, HNO3, and HCl were 49.6, 8.6, 34.2, 21.5, 3.6, 9.8, and 1.4 μgm-3, respectively; and 2.1, 1.5, 2.9, 1.1, 13.1, 12.9, 19.7, and 1.3 μgm-3 were observed for Na+, K+, Ca2+, Mg2+, NH4 +, NO3 -, SO4 2-, and Cl-, respectively, in PM2.5. Levels of O3, NH3, HNO2, HNO3, PM2.5, K+, Ca2+, Mg2+, NO3 -, and SO4 2- showed significant diurnal variations at 95% confidence level. However, SO2, NO2, HCl, Na+, NH4 +, and Cl- did not show any significant diurnal variations. Ion balance showed that an abundant quantity of NH3 was present in the study area to neutralize H2SO4, HNO3, and HCl to form (NH4)2SO4, NH4NO3, and NH4Cl, and thermodynamics for the formation of NH4NO3 was favored under more humid conditions. © 2015 American Society of Civil Engineers.