Block copolymers of aniline (r(1) < 1) and ethylaniline (r(2) > 1) can readily be formed by acid-catalyzed oxidation. These copolymers have enhanced solubility in common solvents, compared to polyaniline. For example, while a 47 mol % o-ethylaniline copolymer is 2-7 times more soluble in ethanol, methanol, and acetone than polyaniline, the solubility of the copolymer in tetrahydrofuran increases by more than 20 times. Thermal effects on the properties of polyaniline and poly(aniline-co-o-ethylaniline) in their emeraldine base forms have been studied by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR), four-probe conductivity, UV-vis, and FT-IR spectroscopies. Upon heating, the number of free electrons in the polymer, as determined by EPR, increases by up to 10 times. An irreversible exothermic transition was observed by DSC, which is believed to be an indication of cross-linking. The electron-donating character of the ethyl group appears to help stabilize the intermediates during the cross-linking reaction. Complete characterization of the copolymer is given.