Careful cleaning of a nickel filament has allowed measurements of the reactivity of its surface towards hydrogen to be made. The initial sticking probability at room temperature was 3.6 × 10–3 and the maximum uptake 1.5 × 1014 mol. cm–2. Both sticking probability and uptake increased at lower filament temperatures. The shape of the sticking probability curve is attributed to the formation of a long-lived precursor to chemisorption.