Dependent on the pH of the aqueous phase, the transfer of substituted P-diketone and 4-acyl-5-pyrazolone anions as well as the transfer of protons facilitated by the ligand anions across a microhole supported water I nitrobenzene microinterface has been studied by cyclic voltammetry. Above a certain pH the half-wave potential of the observed wave is independent of the pH and represents the simple transfer of the ligand anion between the two phases. This critical pi-I-value is dependent on the basicity and the hydrophobicity of the compound. Below this pH-value, the half wave potential is shifted linearly by ca. 52 mV with the pH and reflects the protonation/deprotonation process at the interface. The protonation constants of the ligands in the organic phase, the distribution coefficients, the diffusion coefficients and the formal potentials of the ion transfer were determined. With increasing chain lengths of the acyl-substituent, the formal potential of the transfer of the ligand anions across the interface shifts to more positive values which means that the standard Gibbs energy of the ligand anion partition between water and nitrobenzene decreases. The formal potentials for the 4-acyl-5-pyrazolone anion were found between 50 and -120 mV and the standard Gibbs energies of ion partition between -5 and 12 kJ mol(-1). The protonation constants of the ligands in the organic phase amount to 16.5 +/- 1.0.