1-(2-ethyl-1,2,3,4-tetrahydro-quinazolin-2-yl)-ethanone oxime (ETQX, HL) and the Cu(II) complex of its open-chain tautomer were synthesised and characterised by elemental analysis, magnetic susceptibility, molar conductance, thermal analyses, and spectral (IR, NMR, AAS, LC-MS, and UV-Vis) studies. The accumulated data exhibited that the ligand underwent a ring-opening reaction upon formation of a mononuclear Cu(II) complex in which the chain tautomer acted as an N,N,N-donor. The Cu(II) complex was formed in a 1 : 1 (M : HL') ratio, as indicated by analytical results and the general formula was elucidated to be [Cu(HL')Cl]Cl. The protonation constants of ETQX and the stability constants of the complex species formed with the Cu(II) of ETQX were studied by the potentiometric method at 25A degrees C in 0.1 M KCl ionic medium. A complexation model for the Cu(II)-ETQX system was established with the software program "BEST" based on the potentiometric data. The concentration distribution curves of ETQX and Cu(II)-ETQX complex species in solution were evaluated by the SPE software program. The stoichiometry of the Cu(II) complex in solution was observed to be similar to that of the solid state Cu(II) complex.