A systematic study for the effect of substituents on the thermodynamic stability constant of 1:1 complexes of Cu(ll) with six monosubstituted 2-hydroxybenzoic acids was carried out in aqueous media (deionized water) using Cu(II) ion selective electrode [ CU(LL) ISE ]. Calibration of the electrode with standard solutions of Cu(II) perchlorate ( 10^{–2} to 10^{–6} mol/dm^{3}) using the standard double junction silver / silver chloride reference electrode gave the best Nernstian response ( correlation of emf with pCu ). The test solutions [ containing Cu(ll) ions, ligand and base] were prepared and the equilibrium cell emf for each solution was measured at five temperature. For best precision, optimum conditions were selected (for each test solution) to give a sufficiently higher potential [higher free Cu(ll) activity] than that of the detection limit of the electrode. The equilibrium hydrogen ion concentrations of the test solutions were calculated from the electroneutrality principle and compared with that obtained from measured pH at 25°C. The data (various concentrations used in each test solution and measured emf) were fed into computer programs to perform all the calculations needed as well as those for thermodynamic stability constants and thermodynamic functions “G°, ”H°, and “S°. The high negative values of ”G° indicate that the complexing process is spontaneous and stable. The negative values of “S° show that entropy is not in favor of the reaction to form the complex. However, the highly negative values of ”H° indicate that enthalpy is the strong driving force for the formation of the complexes. A thermodynamic treatment for the substitution has been presented and a correlation between logS (where S is the stability constant of the complex) and pK_{2} (where K_{2} is the second ionization constant of the acid) gave a good straight line whose correlation coefficient, slope, intercept and standard deviation in logS are 0.994, 0.8005±0.044, 2.1454±0.575 and 0.00109 respectively. Since the slope is 0.8005, it is concluded that substitution affects the Cu(ll) ion in these complexes to become as a À-electron doner.