3-alkoxymethyl-3-tert-butyl-4-(3-tert-butyldimethylsiloxy)phenyl-4-methoxy-1,2-dioxetanes (2) with TBAF in DMSO gives intensive blue light emission (λmax = 463nm, ΦCL > 0.1) with shorthalf-life (t1/2 < 1 sec), whereas the methylene analogues (6) cause chemiluminescence with far longer half-life. A dioxetane (11) bearing a phosphoryloxyphenyl is cleaved enzymatically by alkaline phosphatase to generate light with high