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RATIONALE Outdoor particulate and gaseous air pollutants impair respiratory health in children and these associations may be influenced by particle composition. OBJECTIVES To examine whether associations between short-term variations in fine particulate air pollution, oxidant gases, and respiratory hospitalizations in children are modified by particle constituents (metals, sulfur) or oxidative potential. METHODS We conducted a case-crossover study of 10,500 children (0-17 years of age) across Canada. Daily fine particle mass concentrations and oxidant gases (nitrogen dioxide, ozone) were collected from ground monitors. Monthly estimates of fine particle constituents (metals, sulfur) and oxidative potential were also measured. Conditional logistic regression models were used to estimate associations between air pollutants and respiratory hospitalizations, above and below median values for particle constituents and oxidative potential. MEASUREMENTS AND MAIN RESULTS Lag-1 fine particulate matter mass concentrations were not associated with respiratory hospitalizations (odds ratio and 95% confidence interval per 10 μg/m3 increase in fine particulate matter: 1.004 [0.955, 1.056]) in analyses ignoring particle constituents and oxidative potential. However, when models were examined above/below median metals, sulfur, and oxidative potential, positive associations were observed above the median. For example, the odds ratio and 95% confidence interval per 10 μg/m3 increase in fine particulate matter was 1.084 [1.007, 1.167] when copper was above the median, and 0.970 [0.929, 1.014] when copper was below the median. Similar trends were observed for oxidant gases. CONCLUSIONS Stronger associations were observed between outdoor fine particles, oxidant gases, and respiratory hospitalizations in children when metals, sulfur and particle oxidative potential were elevated.