An arc model for a high‐pressure sodium lamp is developed which relates the position of the peaks of the self‐absorbed 3S‐3Psodium resonanceDline to the operating densities and pressures of sodium and mercury. Vapor pressure equations for a Na‐Hg amalgam are derived and used to relate the pressures to the temperature and molar ratio of the amalgam reservoir. Allowing for change in amalgam composition due to preferential evaporation of Hg, the amalgam and vapor pressure data are then related to the total amounts of Na and Hg in the arc tube. In order to obtain agreement with experimental data forD‐line widths between 25 and 200 A˚, it is found necessary to maintain the distinction between the two doublets of the NaDline. In addition, it is shown that the effect of the temperature profile is practically negligible in relating the peak shifts to the average Na and Hg densities.