Absolute number densities and the spatial distribution of atomic hydrogen in a large-scale microwave plasma reactor were measured by means of two-photon absorption laser-induced fluorescence (LIF) with laser radiation at 205 nm. The microwave discharge was operated at 2.45 GHz with a maximum power of 2 kW. Absolute number densities were obtained by calibrating the LIF detection system via NO₂ titration in a flow tube reactor and in the range from 0.5×10¹⁵ to 2.5×10¹⁵ cm⁻³. Compared to small-scale reactors with a volume-to-surface ratio less than 3 cm, this reactor has a relatively homogeneous spatial distribution of atomic hydrogen. The mole fraction of atomic hydrogen in hydrogen plasmas ranges from 10 to 20. The gas temperature–volume effect plays an important role in the spatial distribution of atomic hydrogen in this reactor.