A high-pressure Hg discharge with NaI and ScI3 additives is studied through model calculations and spectroscopic diagnostics. The computer model solves the coupled energy, radiation, and chemistry equations for the discharge on a self-consistent basis. The wall-stabilized discharge is assumed to be in local thermodynamic equilibrium and to exhibit radial symmetry and axial homogeneity. From a prescription of the boundary conditions, the model predicts radial variations in discharge quantities, such as the temperature, chemistry, and radiation field. Predictions are checked by directly comparing model results with experimental data for a horizontal discharge tube. The arc tube is rotated about its own axis so as to suppress convective effects. Necessary input to the model includes oscillator strengths and broadening constants for a large number of Hg, Na, and Sc spectral lines, as well as scattering cross sections and thermochemical data. The required data have been gathered from available sources and determined internally from the experiment. The model is used to study mechanisms in the energy balance which contribute to radial spreading of the discharge. In the absence of the NaI and ScI3 additives, resonance radiation transport plays a major role in the spreading of the high-pressure Hg discharge. With the additives, this effect is diminished, and the more uniform ohmic heating associated with the low ionization potential of Na becomes important.