A homologous series of soluble poly-p-phenylenes containing up to eight phenylene moieties (PP2-PP8) with branched iso-alkyl (or bis-n-alkylmethyl) groups has been synthesized and the structure-property relationship with regards to various optical and electronic properties established. All electronic and optoelectronic properties of poly-p-phenylenes followed a 1 In relationship (where n is number of phenylene moieties) with the increasing number of phenylene moieties. The low-energy electronic transition of the PP2-PP7 cation radicals, generated either by laser-flash photolysis or by chemical oxidation, varied as well according to the inverse (1/n) relationship. The observed inverse relationship of all measured electronic and optoelectronic properties against the increasing number of phenylene units in various PPn's, as well as X-ray crystallography of both neutral and a cation-radical salt of a representative tetra-p-phenylene oligomer allows us to demonstrate that the effective conjugation length in poly-p-phenylenes is, in part, controlled by the increasing number of interactions of ortho hydrogens which may prevent simultaneous planarization of the continuous arrays of a large number of phenylene moieties.