Nitrous oxide reacts with F_s⁺(H) centres (surface oxygen ion vacancies with a single trapped electron) to give (O^–)_s⁺ ions adsorbed at the anion vacancy. The reaction, F_s⁺(H)+ N₂O →(O^–)_s⁺+ N₂ is quantitative and the reflectance spectrum indicates a band centred at 2 eV which is ascribed to the (O^–)_s⁺ centre. Comparison of the optical transition energy calculated from the e.s.r. data with the observed transition shows that the normally accepted value of 135 cm^–1 for the spin-orbit coupling of the O^– ion is too low and that a value of >200 cm^–1 is more appropriate for the solid state. Nitrous oxide also reacts with other surface centres to give an adsorbed species which may be (O^2–₂)_s or (O^–₂)_s. The reaction of (O^–)_s⁺ with hydrogen reforms the F_s⁺(H) centre which then reacts with more nitrous oxide; the reaction can be cycled several times. The reactions of (O^–)_s⁺ with O₂, CO₂, alcohols and olefins are also investigated. To facilitate reference to various types of surface defects and adsorbed species a nomenclature is proposed which is compatible with a recently suggested notation for defects in bulk lattice.