Peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) were measured sequentially in situ by an online gas-phase chromatograph with electron capture detector at urban (Peking University, PKU) and suburban (Yufa, A town in the south of Beijing) sites in Beijing during the photochemical season in 2006. Maximum and average values of PAN were 11.22 ppbv and 1.95 ppbv at PKU during 15 to 27 August, and maximum and average values of PPN were 2.51 ppbv and 0.41 ppbv at Yufa during 2 to 12 September. Average mixing ratios (PAN/PPN) were 5.60 (at PKU) and 5.83 (at Yufa), which is much lower than those in other metropolitan areas. High correlation between PAN and PPN reflects similar volatile organic compound origins. Thermal loss of PAN and PPN was remarkable when compared with their ambient concentrations. PAN and PPN with ozone have similar trend by day. Thermal decompositions of PAN and PPN were calculated, and results indicated that thermal losses influence their atmospheric lifetime significantly. The percentages of PAN loss at the two sites were very similar; however, PPN urban loss was higher than that in suburban