The H + NO2 titration scheme for the determination of atomic hydrogen densities within a microwave excited flow tube reactor has been investigated by laser-induced fluorescence spectroscopy in the vacuum UV. Absolute hydrogen densities are determined on the basis of calibration by Rayleigh scattering from argon. The measurement is performed at a gas mixture containing 0.5% of D-2 added to the main gas H-2. The ground state density of the hydrogen atoms generated in the flow tube reactor was inferred from the fluorescence radiation of the spectrally shifted optically thin D-Lyman-alpha transition. 2002 Published by Elsevier Science B.V.