In this study, acid dissociation constants of 5,5-diphenylpyrrolidine N-aroylthiourea derivatives, exhibiting anti(myco)bacterial activity, were determined by potentiometric titration in 30% (v/v) acetonitrile-water hydroorganic solventat 25 ± 0.1 °C, at an ionic background of 0.1 mol / L of NaCl using the HYPERQUAD computer program. Three acid dissociation constants were determined for each compound 1a-e and we suggest that these acid dissociation constants are related to the carboxyl, enol and enthiol groups. Stability constants of their Pt(II) and Ni(II) complexes were also determined by potentiometric titration under the same conditions stated above using the HYPERQUAD computer program. The ligands behave as bidentate and bind to the metal atom via the S and O atoms. In various pH conditions, the different complex forms were formulated as ML2, MHL2, MH2L2, MH3L2, MH4L2, MH5L2 and MH-4L2 between Pt 2+ /Ni 2+ ions and 5,5-diphenylpyrrolidine N-aroylthioureas. Stability constants of the complexes show that the ligands 1a-e form complexes with Pt(II) and Ni(II) metals as 2:1. The stability of the complexes formed by the ligands and platinum were found to follow the order: 3d > 3a > 3e > 3c > 3b. The stability of the complexes formed by the ligands and nickel were found to follow the order: 4d > 4c > 4a > 4b > 4e. As a result, generally the platinum complexes formed with these ligands are more stable than their nickel complexes.