Novel 5,5-diphenylpyrrolidine N-aroylthioureas, containing 4-methylbenzoyl, 2chlorobenzoyl, 2,4-dichlorobenzoyl, and 2-naphthoyl were synthesized, and their structural analyses were performed using 1 H Nuclear Magnetic Resonance (NMR), 13 C NMR, Fourier transform infrared spectroscopy (FT-IR), mass spectrometry (MS), and high-resolution MS (HRMS) techniques. The acid dissociation constants of the 5,5-diphenylpyrrolidine Naroylthiourea derivative compounds were determined using Hyperquad computer program for data obtained using potentiometric titration method in 25% (v/v) dimethyl sulfoxide-water hydro-organic solvent in the presence of 0.1 molL 1 ionic strength of NaCl and in the acidic medium at 25±0.1 °C, using sodium hydroxide base as a titrant. Two acid dissociation constants were obtained for 3b and 3d, and it was suggested that they were related to N-H and enol groups. Furthermore, three acid dissociation constants obtained for 3a indicated that they were related to N-H, enthiol, and enol groups, and four acid dissociation constants obtained for 3c suggested that they were related to N-H, enthiol, enol, and carboxyl groups.