In the laccase-catalyzed polymerization of m-phenylendiamine in 100% aqueous buffers, the yield of the polymer was strongly influenced by various reaction conditions such as the solution pH and the concentrations of laccase and m-phenylendiamine. When the reaction was performed at pH 3, the 100% synthetic yield of the polymer was achieved. As pH increased, the yield of the polymer decreased significantly to only 4.4% at pH 9. Effects of solution pH on the morphology and the thermal stability of the polymer were investigated in detail. The polymer synthesized at pH 3 has the typical aggregated morphology of globular particles, but being synthesized at pH 7, it has non-aggregated morphology. The thermal stability of the polymer deteriorated as reaction pH increased.