8-Benzoyl-9-deuterio-naphtho [de-2.3.4]bicyclo [3.2.2]nona-2,6,8-triene (1) rearranged quantitatively in a photochemical di-π-methane-type process to 2-, 6-, and 9-deuteriated 1-benzoyl-naphtho [de-2.3.4]tricyclo [4.3.0.02,9]nona-2, 6-diene (8a–c). The phenylhydroxymethyl analogue 2 underwent a similar regioselective rearrangement to 9a–c. The rearrangement 1 → 8a–c is proposed to proceed along three reaction paths evolving from two primary photochemical processes of naphthylvinyl and vinyl-vinyl bonding (1 → 3 + 6). Evidence for a competition between several paths and involvement of biradical intermediates derives from changes in the isotopomeric composition with temperature, and from laser flash detection (λexc 353 nm) of a transient. The dependence of the quantum yield for product formation from 1 on excitation wavelength and sensitizer triplet energy leads to the conclusion that reaction to the primary biradicals occurs directly from the S1 (n, π*) and T2 (n, π* ) states, and that reaction from T1 (π, π*) and from S2 (π, π*) proceed either directly or via T2.