Display omitted •The first study focusing on HO and O2− generation in the CaO2/Fe(II) system.•Reactive oxygen species and their generation pathways were proposed.•The strategy enhancing HO generation efficiency was developed.•A concept model using CaO2/Fe(II) oxidation technique was addressed. Calcium peroxide (CaO2) is a stable hydrogen peroxide (H2O2) carrier, and the CaO2/Fe(II) system has been applied for treatment of various pollutants. It is commonly reported in the literature that hydroxyl radical (HO) and superoxide radical anions (O2−) are the two main reactive oxygen species (ROSs) generated in the CaO2/Fe(II) system. However, many of the reported results were deduced from degradation performance rather than specific testing of radical generation. Thus, the specific generation of ROSs and the influence of system conditions on ROSs yield are still unclear. To our knowledge, this is the first study specifically focusing on the generation of HO and O2− in the CaO2/Fe(II) system. Experimental conditions were optimized to investigate the production of HO and O2−. The results showed the influences of CaO2, Fe(II), and solution pH on HO and O2− generation, and the HO generation efficiency was reported for the first time. In addition, the ROSs generation pathways in the CaO2/Fe(II) system were elucidated. A strategy for enhancing HO yield is developed, based on the continuously dosing Fe(II). This proposed strategy has implications for the effective application of in situ chemical oxidation employing CaO2/Fe(II) for groundwater remediation.