A novel calibration method for the determination of absolute species densities by laser spectroscopy with two-photon absorption is presented. The method is based on a comparative measurement with a noble gas which has a two-photon resonance spectrally close to the transition investigated. Application of this scheme for the calibration of LIF measurements to determine atomic oxygen, generated in a capacitively coupled rf reactor, and for calibration of a RIS measurement of atomic oxygen and carbon generated by sputtering is demonstrated. The relevant excitation cross section of the involved 7p[3/2](2) xenon state, which is used for calibration of atomic oxygen densities, is determined.