The invention describes a high intensity gas-discharge lamp comprising a discharge vessel (5, 5′) enclosing a fill gas in a discharge chamber (2) and comprising a pair of electrodes (3, 3′, 4, 4′) extending into the discharge chamber (2), and wherein the fill gas includes a halide composition comprising a halide of sodium and, optionally, scandium iodide to a total proportion of at least 30 wt %, and a halide of terbium and/or gadolinium to a proportion of at least 5 wt %.