The complex formations of N-acetylcysteine with divalent transition metals Cu2+, Zn2+, Ni2+, Co2+, Mn2+ have been investigated in aqueous solution of 0.15 mol.dm(-3) NaCl at 310.15 K by the potentiometric method. The protonation and stability constant of the complexes were obtained based on nonlinear least-squares curve fitting performed by the HYPER-QUAD2008 program along with an estimation using Bjerrum-Calvin half integral method. Distribution diagrams of complex species at certain pH range were generated by the HySS2009 simulation program. From optimization and thermochemical analysis by using the Gaussian09W program, it showed that the ligand formed a six-membered chelate ring with the investigated metal ions via its sulfur and oxygen donor atoms.