Results of experimental determinations of the gas temperature and plasma parameters in a microwave discharge not at thermal equilibrium are presented. The investigation is concerned with high-pressure hydrogen under conditions such that radiation is emitted both by atomic and by molecular components of the plasma are primarily governed by the interaction of the excited atoms and molecules with heavy particles. Because this takes place the approximate models which are successfully applied to analyse data from spectroscopic diagnostics cannot be used to describe the radiative properties of the discharge. One of the main objectives of research consists of making headway in spectral methods of gas-temperature determination in the high pressure range by invoking for interpretation of experimental data calculations performed in the framework of the kinetic scheme of thermal non-equilibrium discharge in hydrogen as well as general physical reasoning relating to the role of interactions between heavy particles in non-equilibrium plasma radiation. The following measurement techniques have been used: spectral measurements of Balmer-series radiation; spectral measurement of the gas temperature by recording the Doppler broadening of the ; and spectral measurements of by recording the Fulcher band. In addition the electron concentration has been measured with the help of a microwave interferometer and pyrometric measurements of the temperature of a test solid body placed in a microwave-discharge plasma have been performed. Measurements of the gas temperature agree satisfactorily under the assumption that collisions of excited particles with heavy components of a gas-discharge medium are of considerable importance. Conclusions regarding the ion composition and degree of dissociation of hydrogen molecules can be drawn.