Abstract A π-electronic method, adapted to the study of non-planar unsaturated molecules, is used to examine the consequences of twisting about the exocyclic bond in derivatives of fulvene and fulvalene. The geometry, electronic spectrum, ionization and reduction potentials of fulvene, 6,6-disubstituted fulvenes, fulvalene, octachlorofulvalene, tetrabenzofulvalene and (9-fluorenylidene)cyclopentadiene are considered. The last-mentioned molecule and its dimer have been prepared and their properties are reported. Incidental comments on the applicability of the Huckel rule (“4n + 2”) to annelated odd-membered rings and on charge-distribution in fused non-alternants are also made.