studied with circularly polarized light in both absorption and emission. Here we describe a comprehensive investigation on the structure and optical properties of poly3,4-bis[( S )-2-methylbutoxy]thiophene} (PBMBT) as an example to elucidate the molecular origin of the chiroptical effects in chiral conjugated polymers. The chiral side chains of PBMBT induce very strong bisignate circulardichroism (CD) effects