The reaction of the methyl-substituted 1,2-dioxetanes 1-3 with triphenylalkylidenephosphoranes 4-7 was investigated. By nucleophilic attack of the negatively charged phosphorane carbon atom at the peroxide bond of the 1,2-dioxetanes, the dipolar phosphonium alkoxides 8-13 were formed, which were in equilibrium with the ring-closed 2,2,2-triphenyl-1,4-dioxa-2 lambda(5)-phosphorinanes 14-19. Hydrolysis of the phosphonium alkoxides 8-12 afforded the phosphine oxides 21-25 by benzene elimination. For the phenyl-substituted phosphonium alkoxide 13, hydrolysis led to the benzyl ether 26, while deprotonation at the cu carbon atom resulted in the corresponding ylide, which on air oxidation led to benzoate 27 and triphenylphosphine oxide. In the presence of benzaldehyde, the ylide derived from phosphonium alkoxide 8 was trapped in the form of the novel 3-hydroxy enol ether 28 through stereoselective Wittig olefination, which establishes the phosphonium alkoxides 8-13 as potentially useful in situ building blocks.