A novel tetradentate N2O2-type Schiff base, synthesized from 1,2-bis(m-aminophenoxy)ethane and 2-hydroxynaphthalin-1-carbaldehyde, forms stable complexes with transition metal ions such as Cu(II), VO(IV) and Zn(II) in DMF. Microanalytical data, elemental analysis, magnetic measurements, UV, visible and IR spectra as well as conductance measurements were used to confirm the structures. The stability constants of these complexes in 60% (v/v) DMF-water were determined at different ionic strengths (0.07, 0.13, 0.2 M) and at different temperatures (45,50,55,60 +/- 0.1 degrees C) using a spectrophotometric method. From these constants, thermodynamic stability constants and thermodynamic parameters (Delta G(0), Delta H degrees, Delta S degrees) were calculated. The values of enthalpy change are negative for all systems. The acid dissociation constant of the ligand, investigated in 60% (v/v) DMF-water, has also been calculated at different temperatures.