Green-emitting poly [(2-alkoxy-5-methyl-1,3-phenylenevinylene)-alt-(1,4-phenylenevinylene)]s (2) were synthesized via the Wittig-Horner reaction. The polymers were yellow resins with the molecular weights as high as 54 600. On the basis of FT-IR and H-1 NMR spectra, the olefins in the polymers were confirmed to be predominantly in the trans configuration. UV-vis absorption of 2 (lambda(max) approximate to 374 - 377 nm) was about 20 nm blue-shifted from that of poly [(1,3-phenylenevinylene)-alt-(2,5-dialkoxy-1,4-phenylenevinylene)]s (1), although the chromophores in the former had the same number of alkoxy substituents as in the latter, Comparison of 2 with the model compound 7 showed that the shorter conjugation length observed from 2 (relative to 1) was the intrinsic property of the chromophore. Polymer 2 also exhibited a higher photoluminescence (PL) than I in both solution and film states, indicating the effect of the substitution pattern on the optical properties of the polymers. The vibronic structures were assigned with the aids of the low-temperature UV-vis and fluorescence spectroscopy. LED devices using 2 gave green EL output (emission lambda(max) at 496 and 520 nm) with the external quantum efficiency of 0.082%.