Dependent on the pH of the aqueous phase, the transfer of protonated forms of 2-benzoylpyridine N(4)-phenyl thiosemicarbazone (BPPT) (which has antimicrobial, antifungal and anticytotoxic activities) and 2-benzoylpyridine N(4)-ethyl thiosemicarbazone (BPET) across water/1,2dichloroethane (1,2-DCE) interface has been studied by cyclic voltarnmetry. The protonation constants of the ligands (pK(a1)(w) and pK(a2)(w)) were a I a2 determined by spectrophotometry. The standard partition coefficients (log P-i(0)) and the standard Gibbs energies of ionic (cationic) species of ligands (Delta G(tr,i)(0,w -> o)) were calculated from the standard transfer potentials (Delta(w)(o)phi(0)(i)). The standard Gibbs energies of their transfer (Delta G(tr,i)(0,w -> o)) and partition coefficients of neutral species (log P-N) were determined by shake-flask method. These thermodynamic parameters were evaluated as a quantitative and qualitative measure of the lipophilicities of two compounds. The differences between the partition coefficients of cationic and neutral form of compounds [diff(log P1+-N)] were interpreted by results obtained from voltammetric data. Effect of N(4)-phenyl and ethyl groups for transfer of 2-benzoylpyridine thiosemicarbazone derivatives at macro-liquid/liquid interface was investigated. The antimicrobial activity of BPET was tested against four types of bacteria and found to be active against Staphlylococcus aureus. (c) 2007 Elsevier Ltd. All rights reserved.