In this paper we present the synthesis, the characterization, and the use in light-emitting diodes of a new blue-light-emitting copolymer, poly[2,5,2",5"-tetraoctyl-p-terphenyl-4,4"-ylenevinylene-p-phenylenevinylene]. This copolymer, obtained by a poly-Heck reaction, has a fully unsaturated backbone consisting of regularly alternating terphenylene and phenylenebisvinylene blocks. The presence of well-defined chromophores, resulting from steric interactions in the polymer chain, gives rise to bright blue fluorescence, both in solution and in thin solid films. Blue-light-emitting diodes were fabricated by using this copolymer as an emitter layer.