A method for measuring the density of mercury in high intensity discharge (HID) lamps using x‐ray absorption is described. The method is suitable for use with lamps with any outer envelopes. Time dependent values of absolute density and cold spot temperature were obtained for a variety of HID lamp types during the period following the extinguishing of the arc. Values for the time dependent mercury mass per unit area at various locations in the lamp were obtained during the warming up of the lamps.