The reaction of aniline with copper(II) perchlorate in acetonitrile yields highly-electroconductive polyaniline perchlorate, [(-C6H4NH-)(ClO4)m·nH2O]x, which is soluble in dimethylsulfoxide (DMSO) and has low degrees of branching and/or crosslinking. The conductivity of a material with m = 0.43 is 3.1 S cm-1at room temperature. The solution electronic spectrum of the perchlorate exhibits a strong band at 285 nm and a weak band at 445 nm; no band is observed in the region of 830 nm. In the solution electronic spectrum of the corresponding polymer base, strong bands are observed at 630 and 325 nm. When the solution of the polymer base is acidified, the 630 nm band disappears and a new band appears at 830 nm: the protonation of the polymer base by the acidification yields a spectrum that is different from that of the original polymer perchlorate. In the polymer chains of the perchlorate prepared by the present method, the majority of positive charges are centred on anilinium ion radicals (polarons) rather than on quinoneiminium ions (bipolarons). © 1989.