Light-induced electron spin resonance (LESR) measurements have been performed on two dialkoxy derivatives (MEH-PPV and CN-PPV) of poly(p-phenylenevinylene) where cyano groups are attached at the vinylic carbon sites in CN-PPV, The transient response and excitation spectra of the LESR signals indicate that the observed spins are optically generated polarons, A clear reduction of ESR linewidth has been observed in CN-PPV compared with MEH-PPV, providing direct evidence that the polaron spin density resides predominantly on the vinylic carbons. Linewidth analysis based on a theoretical model for the spin distribution yields an extension of the polaron over approximately four phenyl rings.