Abstract [F A] 0 centres in ZnO and ZnS single crystals have been detected by electron paramagnetic resonance. The [F A] 0 centre consists of an anion vacancy with a localized electron and a monovalent substitutional impurity replacing an adjacent Zn 2+ ion. This impurity is probably a Li + ion in the case of ZnO and a Cu + ion in the case of ZnS. The values obtained for the spin Hamiltonian parameters of the [F A] 0 centre in ZnO at 60 K are: g ∥ = 1.9951, g ⊥ = 1.9956, A ∥ = 2.36 MHz and A ⊥ = 2.75 MHz. In ZnS at 30 K the corresponding values are g ∥ = g ⊥ = 2.0017, A ∥ = 4.06 MHz and A ⊥ = 3.67 MHz. A thermallystimulated ionic process occurs in ZnO at 530–650 K and in ZnS at 720–820 K. In the first stage of this process the anion vacancies of the regular lattice detected as F + centres disappear and, in the second stage the destruction of [F A] 0 centres occurs.