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The facts that the halogen becomes affected in this manner only when it is in Rings B or C, and therein only when in Position 3, are interpreted as indicating that of the four rings in the acridyls only these two are capable of undergoing a tautomeric change from benzenoid (VII) to the more reactive quinonoid state (VIII). The conclusion is drawn that in the acridyls, in common with other free radicals, the acquisition of the property of selective absorption on the one hand, and on the other, their capacity to exist tautomerically in the quinonoid state, are concomitant phenomena.