Four bicyclic dioxetanes bearing a phenolic substituent, 3-tert-butyldimethylsiloxy-4-chlorophenyl (3a), 5-tert-butyldimethylsiloxy-4-chloro-2-ethylphenyl (3b), 5-tert-butyldimethylsiloxy-2-ethylphenyl (3c), and 3-tert-butyldimethylsiloxy-4-ethylphenyl (3d), were synthesized. All dioxetanes 3a–3d gave intense blue light on treatment with tetrabutylammonium fluoride (TBAF) in DMSO or acetonitrile. Kinetic study on the fluoride-induced CIEEL decay of these dioxetanes 3a–3d and the parent dioxetane 2b revealed that the para-substitution with chlorine on the phenolic moiety of dioxetane increases free energy of activation (ΔG‡), while the para-substitution with ethyl on the aryl decreases ΔG‡. On the other hand, substitution with an ethyl at the ortho-position instead of the para-position was found to increase ΔG‡ and to suppress the CIEEL decay. This fact is attributed to the steric factor of the ortho-ethyl group which would prevent the aromatic ring from rotating freely around the axis joined to the peroxide ring, and supports the suggestion for a CIEEL-active dioxetane bearing a phenolic moiety that an intramolecular electron transfer occurs preferentially from the phenolic donor to O–O of the dioxetane ring, when the aromatic ring lies in a certain conformation(s). Copyright © 2002 John Wiley & Sons, Ltd.