The effect of oxygen stoichiometry on the luminescent properties of the scheelite-structure tungstates CaWO4 and NaLa(WO4)2 is studied. The oxygen content of the tungstates was varied by annealing in controlled oxygen atmospheres or heterovalent doping. The nature of the centers responsible for the steady-state luminescence of the tungstates is elucidated. The major emission band is tentatively attributed to WO4 regular tetrahedra, and the longer wavelength emission to WO3 · V'O' oxygen-deficient complexes.