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Mass and energy spatial flows within high-pressure lighting plasmas containing metal halides may affect the plasma state of equilibrium. In this study, the effect of the thermochemical equilibrium assumptions on the determination of the plasma parameters in a sodium-iodide arc lamp is experimentally investigated. The electron temperature and electron, mercury-ion, and sodium-ion density as a function of the radial distance from the arc axis in a sodium-iodide arc lamp are determined using an emission spectroscopy technique, which is free of equilibrium assumptions. Comparison of the results with those assuming thermochemical equilibrium shows that the actual plasma state differs from that at equilibrium. Possible factors causing the observed differences are discussed.