Optical emission and absorption spectroscopy and double Langmuir probe diagnostics have been applied to measure the plasma parameters of an expanding magnetized hydrogen plasma jet. The rotational temperature of the excited state H2(d2Πu) has been determined by analyzing the intensity distribution of the spectral lines of the Fulcher‐α system of H2. The gas temperature in the plasma, which is twice the value of the rotational temperature is equal to ≂ 520 K. Several clear indications of presence of the ‘‘hot’’ electrons have been observed in the plasma: (1) Langmuir probe measurements (Te≂1.4 eV), (2) appearance of the Fulcher‐α system of H2 (excitation potential ΔE=13.87 eV), (3) low rotational temperature (T*rot≂260 K) of the excited H2(d3Πu) molecules, (4) local excitation in the plasma of Ar I(ΔE=15.45 eV), and Ar II(ΔE=19.68 eV) spectral lines, (5) local excitation in the plasma of He I(ΔE=23.07 eV and ΔE=24.04 eV) spectral lines. Optical actinometry has been applied to measure the absolute density of hydrogen atoms and hydrogen dissociation degree in the plasma. The measured absolute density of hydrogen atoms are in the (1–1.4)×1020 m-3 range, and the corresponding dissociation degree of the hydrogen plasma is in the range of 8%–13%. © 1996 American Institute of Physics.