The authors conducted a time-series analysis to examine seasonal variation of mortality risk in association with particulate matter less than 2.5 mu m in aerodynamic diameter (PM2.5) and chemical species in Xi'an, China, using daily air pollution and all-cause and cause-specific mortality data (2004-2008). Poisson regression incorporating natural splines was used to estimate mortality risks of PM2.5 and its chemical components, adjusting for day of the week, time trend, and meteorologic effects. Increases of 2.29% (95% confidence interval: 0.83, 3.76) for all-cause mortality and 3.08% (95% confidence interval: 0.94, 5.26) for cardiovascular mortality were associated with an interquartile range increase of 103.0 mu g/m(3) in lagged 1-2 day PM2.5 exposure. Stronger effects were observed for the elderly (>= 65 years), males, and cardiovascular diseases groups. Secondary components (sulfate and ammonium), combustion species (elemental carbon, sulfur, chlorine), and transition metals (chromium, lead, nickel, and zinc) appeared most responsible for increased risk, particularly in the cold months. The authors concluded that differential association patterns observed across species and seasons indicated that PM2.5-related effects might not be sufficiently explained by PM2.5 mass alone. Future research is needed to examine spatial and temporal varying factors that might play important roles in modifying the PM2.5-mortality association.