The acid-base complexation of poly((4-carboxyphenyl)acetylene) with an optically active amine [(R)-(+)-1-(1-naphthyl)ethylamine] not only induces strong circular dichroism (CD) but also changes the molecular weight dependence of the intrinsic viscosity in dimethyl sulfoxide. From the intrinsic viscosity data, we have determined the chain stiffness parameter (the persistence length) and have estimated from this parameter the average standard deviation of the internal rotation angle around the single or double bond of the polyacetylene chain to be 21 and 15degrees before and after the complexation, respectively. The reduction of the torsional fluctuation may arise from the steric hindrance among the amines complexed with the carboxy pendants which makes the internal-rotation energy well steeper for the polyacetylene backbone. The reduction is however not as remarkable as the change in CD due to the complexation. We conclude that the remarkable CD behavior arises from a drastic change in population of the right- and left-handed helices of the polymer, and the helical conformation of the complex is still imperfect.