A radical source was developed and tested that allows one to generate HO2 at atmospheric concentrations. It is based on H2O photolysis at 185 nm with subsequent conversion of OH and H to HO2 for radical production and photolysis of O2 at the same wavelength to produce a reference ozone concentration. The source can be used as an absolute calibration method for chemical amplifiers by linking the produced HO2 concentration to the ozone concentration. It can also be used to produce organic alkyl peroxy radicals, if CO is replaced by the respective hydrocarbons. The source was calibrated with matrix isolation-electron spin resonance spectroscopy. The measured concentrations are in good agreement with those calculated from first principles if losses due to the self reaction of HO2 are taken into account. These losses are less than 10% at HO2 concentrations below 100 parts per trillion by volume (pptv) and increase to about 40% at an HO2 concentration of 250 pptv. Without addition of CO, the source can also be used for OH calibration, but chemical losses are then more important. ¿ American Geophysical Union 1995