The quenching of singlet oxygen (O-2(1) Delta(g)) by the closed and open forms of various spiroindolinonaphthoxazines has been Investigated by use of pulsed laser techniques. Significant variations in the quenching rate constants have been found depending on the molecular structures of the photochromic compounds. In addition, experiments which attempt to resolve the contributions to singlet oxygen quenching by the closed and open forms of the photochromics are discussed. It is demonstrated that the open form exhibits enhanced singlet oxygen quenching relative to the closed form.