A method is in development which should allow the determination of absolute number densities of H and 0 atoms in flames from two-photon laser-excited fluorescence measurements. The calibration will be based on reference experiments in a discharge-flow reactor. In this study, the influence of fluorescence quenching and the generation of additional H and 0 atoms by laser photolysis of the flame gases is investigated. Consequences for typical hydrogen-oxygen flame conditions are discussed.