Molecular interactions in polypyrrole (PPy) doped with dodecylbenzene sulfonic acid anion were studied as a key factor. affecting solubility of thr polymer. The bulky dopant molecules are able to restrain the molecular interactions in as-polymerized PPy, but molecular interaction increases gradually as time elapses, causing insolubilization of PPy powder. Insolubilization results mainly from a formation of crosslinks between polymer chains.