This study was aimed to investigate the concentration of hydrogen peroxide (H2O2), nitric acid (HNO3) and reactive nitrogen (NOy) at three sites in Southern Taiwan during 2003–2004. The mean H2O2 concentrations were 1.0–2.7 ppbv, 1.6–2.8 ppbv and 1.0–2.6 ppbv at Pingtung, Chao-Chou and Kenting, respectively. Meanwhile, the mean HNO3 concentrations were 2.7–5.5 ppbv at Pingtung, 1.9–4.6 ppbv at Chao-Chou and 1.2–3.0 ppbv at Kenting. The NOy mean concentrations were 61.0–85.0 ppbv, 16.0–75.0 ppbv and 4.2–12.0 ppbv at Pingtung, Chao-Chou and Kenting, respectively. The hourly data of H2O2, HNO3, NOy and O3 were then used to determine the ratio of H2O2 to HNO3, O3 to HNO3, O3 to NOy and the extent of reaction (E) in Smog Production Model (SPM) to evaluate the NOx and VOC sensitivity of ozone formation in Southern Taiwan. The results show that Pingtung city was primarily VOC-sensitive regime in four seasons. At Chao-Chou, the percentages of VOC-sensitive regime exceed those in NOx-sensitive regime in H2O2/HNO3 and O3/NOy in four seasons; but O3/HNO3 indicates NOx-sensitive regime in autumn, winter and spring. Kenting was mainly dominated by NOx-sensitive by use of three indicators in four seasons. The SPM results are consistent with those determined by the photochemical indicator ratios although the percentages of dominance may vary among these analyses.