Light-induced ESR (LESR) has been employed to study optically-generated polarons in two dialkoxy derivatives of poly(p-phenylene vinylene) (PPV), MEH-PPV and CN-PPV. A clear reduction of LESR linewidth is observed in CN-PPV compared with MEH-PPV and also with PPV previously reported, providing direct evidence that the polaron spin density predominantly reside on vinyl sites. This feature can be reproduced by the linewidth analysis based on a theoretical spin distribution of a polaron extending over approximately 4 phenyl rings.