We describe an experimental setup in which resonance‐enhanced multiphoton ionization (REMPI) is used for diagnosis of neutral particles (atoms or molecules) effusing from a plasma. Quantum‐state‐specific detection, achieved by selective ionization with the help of REMPI, yields the distribution over their internal degrees of freedom. Discrimination against nonlaser‐produced ions is performed by means of the combination of an electric and a magnetic field. The applicability of the setup is demonstrated by the determination of the rotational temperature of ground‐state hydrogen molecules in a multicusp ion source. Under nondischarge conditions a detection limit is achieved of ≊2×1010state‐selected hydrogen molecules/cm3, corresponding to a value of ≊7×108/cm3at the analysis region. Under discharge conditions, this detection limit is deteriorated by the presence of metastable hydrogen molecules, which are ionized in a one‐photon process.