Two-photon laser-induced fluorescence spectroscopy is used to detect ground-state atomic hydrogen in highly diluted SiH4–H2 radio-frequency discharges currently used for microcrystalline silicon thin film deposition. Along with the fluorescence coming from the hydrogen atoms created in the discharge, additional atoms arising from photodissociation of silicon containing species at 205 nm are observed. This parasitic effect is identified as resulting mainly from the disilane molecules formed in the plasma. Hydrogen atoms created in the plasma and originating from the photodissociation are discriminated by a spectral analysis of the laser-induced fluorescence line profile and relative densities are deduced. Some results on the hydrogen atom and disilane concentrations as functions of the silane dilution, radio-frequency power, gas pressure, and gas temperature are given to illustrate the potentiality of such a diagnostic in SiH4–H2 discharges containing up to 6% of silane. © 2002 American Institute of Physics.