A new attempt has been made to identify the factors responsible for the abnormally small one-bond C, H coupling consts. obsd. for allyl, pentadienyl and heptatrienyl compds. of Li and K. In-plane deformations (widening of the central CCC angle) should have only a small, if not negligible effect. The out-of-plane bending of hydrogen atoms appears to contribute significantly, as probably also does accumulation or polarization of electron d. in conjugated structures. [on SciFinder (R)]