Spatially resolved arc temperatures and sodium density measurements are presented for two high‐pressure sodium arcs. Absolute intensities of the optically thick 818/819‐nm lines were used to determine the arc temperature while the radial profile of the optically thin 514‐nm line was Abel inverted to determine the Na atomic density. Agreement with an independent measurement of the Na density obtained by controlling pressure of sodium in the lamp with a tin bath consistently required a value for the Na 514‐nm transition probability 2–3 times smaller than the literature value (A=0.011×108s−1) of Wiese, Smith, and Miles [AtomicTransitionProbabilities, NSRDS‐NBS 4 (NBS, Washington, DC, 1971), Vol. II], obtained from quantum mechanical calculations. The results of three separate experiments indicate that the value should be modified toA=0.0040×108s−1with a standard deviation of ±21%. A more detailed error analysis including systematic error would indicate an accuracy to within ±33%.