Although 1-(o-tolyl)1 -benzoylcyclopropane (I) acid 2-(o-tolyl)-2-benzoyloxirane (II) are structurally analogous, they go through dramatically different reaction pathways upon absorption of photons. I yields a single photoproduct, while H-2(2)-substituted II gives birth to three different photoproducts. The structural and kinetic features of the entire reaction intermediates have been measured with laser flash techniques, I at T-1 undergoes intramolecular hydrogen transfer at 2.3 x 10(8) s(-1) from the methyl group to the carbonyl group to form a biradical intermediate, which transforms into a cyclized photoproduct (Phi, 0.14; 5 x 10(7) s(-1)). However, the carbonyl group of II at T1 can abstract a hydrogen atom from the oxiranyl ring (6 x 10(7) s(-1)) as well as from the methyl group (2.3 x 108 s(-1)). The biradical intermediate from the former process immediately rearranges into a photoproduct (Q, 0.025), while the one from the latter transforms into a new oxiranyl ring-opened intermediate (1.1 x 10(8) s(-1)) or a cyclized photoproduct (Phi 0.075; 3.3 x 10(8) s(-1)). The new intermediate rearranges into another photoproduct (Phi 0.025; 3.7 x 10(7) s(-1)). The dramatic changes of II are theoretically attributed to the drastic increase in the acidity and instability of the three-membered ring with oxygen substitution.