A series of stereoisomeric 9-substituted hexahydro-1,4- methanobiphenylenes, (4)-(10),were synthesized and their products and relative rates of bromination (Br₂/AcOH) determined. Bromination occurred exclusively at the 6-position to give compounds (16) and (17). The syn-methyl ether (5) was found to be at least 3 × 10⁵ times more reactive than the corresponding ketone (7) in the exo-fused series. With the aid of INDO MO SCF calculations it was concluded that: (a) the enhanced reactivity of syn-methyl ether (5) compared with the parent hydrocarbon (4) is a consequence of orbital interactions through space; (b) of the 2000-fold diminished reactivity of ketone (7) compared with the parent hydrocarbon (4), a factor of 40 may be attributed to inductive and orbital interactions through bond effects; the residual factor of 50 is due largely to (repulsive) electrostatic field effects operating between the carbonyl carbon atom and the developing positive charge on the aromatic ring during electrophilic attack.