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The combination of reflectance UV-Vis spec-troelectrochemistry with electrochemical quartz crystalmicrobalance (EQCM) and separate reflectance FTIR char-acterization were used to investigate the structural intercon-version for poly(o-phenylenediamine) (PoPD) between its ladder structure with phenazine units and polyaniline-like linear chains. The poly(o-phenylenediamine) films potentio-statically (0.8 V vs. SCE) grew on Au electrodes from 0.20 mol·L-1 H2SO4(PoPD1)or 0.40mol·L-1 NaOH(PoPD2)aqueous solution containing 0.20 mol·L-1 Na2SO4+0.10mol·L-1o-phenylenediamine. By considering the mass of deposited PoPD2 film obtained from the EQCM data and the charge consumed under the current peak at ca. 0.6 V vs. SCE for oxidation of -NH2 groups in as-prepared PoPD2 during potential cycling in 0.10mol·L-1 aqueous H2SO4,the nolar percentage of the polyaniline-like chains was estimated to be 19% (relative to total phenylenediamine units), being in agreement with the result obtained from a formaldehyde combination experiment through the aminocarbonyl reaction. After 40-cycle potential sweeps between 0.2 and 0.8 V vs. SCE the polyaniline-like chains in PoPD2 could be completely converted via intramolecular cyclization into the ladder structure with phenazine units. However, PoPD1 was found to be perfectly composed of the ladder structure with phenazine units, and after 40-cycle potential sweeps between -0.4 and 0.1 V vs. SCE only 2.5% in molar percentage of PoPD's ladder structure could be converted into polyaniline-like chains, suggesting that the ladder structure with phenazine units is thermodynamically more stable due to its possessing higher conjugation.