Highly conductive new aniline copolymers containing butylthio substituent have been first prepared from unsubstituted polyaniline, utilizing a concurrent reduction and substitution reaction performed in the solid-state matrix of polyaniline. The resultant copolymer film after one reaction treatment cycle was found to contain ca. 38 mol % of butylthio group and was as conductive (3-4 S/cm) as its parent polyaniline film (2-3 S/cm). Most surprisingly, copolymers containing butylthio group as high as 91 and 121 mol % (based on atomic ratio S/N measured by XPS) can also be easily prepared and still displayed a conductivity as high as 1 and 0.6 S/cm, respectively, despite the presence of the large amount of bulky butylthio substituent. All these new butylthioaniline copolymers are highly soluble in THF, dioxane, 2-methoxyethyl ether, and 2-methoxyethanol, which are nonsolvents for the parent polyaniline. The results suggest that the concurrent reduction and substitution route is a better way, as compared with the conventional copolymerization method, for preparing aniline copolymers with a more conjugated and regular backbone structure.