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The molecular weight of polyaniline can be substantially increased from that normally obtained (M w ~ 53 000) by the oxidative polymerization of aniline with ammonium peroxydisulfate at about 0 to 3 o C up to about 218 000 by the presence of selected dissolved neutral salts. A further increase up to M w ~ 385 000 can be obtained by carrying out the polymerization at about -40 o C. Similar results are obtained for the polymerization of o-methoxyaniline to yield a maximum M w ~ 221 000 at 0 to 3 o C and M w ~ 417 000 at about -40 o C. The results are consistent with a reduction in the rate of the initiation step in solution and an increase in the rate of the chain propagation step at the solid polymer/solution interface caused by Donnan equilibrium effects.