Infrared-active SO stretching bands [ν(SO)] observed for the sulfonated polyethylene (PE) samples were found to change their vibrational frequencies systematically depending on the degree of the π-conjugated chain structure generated through the sulfonation reaction and also on the sulfonate ion species SO3X in the structure [−CHCHCHCHCHC(SO3X)CC−...], where X = H, Na, Li, K, Ca, and Zn. For the sulfonated PE samples with SO3Na groups, the ν(SO) band was found to shift to the higher frequency side as the conjugated chain length was increased. When the π-conjugated chain structure was modified by bromination reaction (...−CCCCCCCC−... + Br2 → ...−CCCCCBrCBr−...), such a high-frequency shift was canceled. The sulfonated PE samples with SO3H groups, on the other hand, did not show any high-frequency shift even for longer π-conjugated chain structure but gave the ν(SO) bands at the position rather close to those of the SO3Na species coupled with a short π-conjugated chain. These spectral changes were confirmed also by measuring the spectral changes for a series of the model compounds having the π-conjugated structure combined with the SO3Na groups.