The relative intensities of the Balmer series lines of hydrogen and deuterium atoms and the Q-branch lines in the Fulcher α-band system of the H2, HD, and D2 molecules were measured. These lines were emitted by nonequilibrium plasma surrounded by the cylindrical channel of an additional molybdenum electrode located between the cathode and anode of glow (in the deuterium with a minor hydrogen impurity) and arc (in a mixture of D2, H2, and Ne in comparable quantities) discharges at pressures of 6–8 Torr and current densities of 0.4 and 8.5–17 A/cm2, respectively. The measured intensity ratios and gas temperature were used for the estimation of the relative particle densities of the H2, HD, and D2 molecules with the aid of the simple models of atomic excitation (models 1a and 1b for the high and low degrees of dissociation, respectively) and the rovibronic levels of isotopic molecules (model 2). The results obtained by means of models 1a and 2 were in a significant contradiction, whereas the data obtained with t