The excitation step of the intramolecular CIEEL generation in the triggered cleavage of spiroadamantyl-substituted dioxetane has been studied. The electron back-transfer (BET) process versus the direct chemiexcitation of the phenolate-anion emitter have been considered as mechanistic alternatives. The observed solvent-cage effect on the CIEEL generation, manifested by the increase of the singlet chemiexcitation yield at increased viscosity, provides evidence that the BET process operates in the intramolecular CIEEL mechanism.