An acid dissociation constant (pK_a) of bupropion has been determined in an aqueous solution by capillary zone electrophoresis (CZE). Electrophoretic mobility of bupropion was measured by CZE over its acid-dissociating pH range, and a pK_a value has been determined as 8.75±0.02 (mean ± standard error, ionic strength: 0.010 mol dm⁻³). Although bupropion degrades under heated and alkaline conditions and the degraded species from bupropion coexist in the aqueous solution, the pK_a value of bupropion can also be determined by CZE with the residual bupropion without any interference from the degraded species. The pK_a value determined after the heat-degradation was 8.74±0.02; the result agreed well with the one determined with the freshly prepared solution. Utilization of the separation characteristics of CZE has been demonstrated with bupropion for the analysis of the fast equilibrium of acid-base reaction under the coexistence of the degraded species.