Thermodynamic protonotion constants of N-ortho xylyl thiovioluric acid [NOXTVA], N-para xylyl thiovioluric acid [NPXTVA], N-meta xylyl thiovioluric acid [NMXTVA] and N-á-naphtyl thiovioluric acid [NANTVA] and thermodynamic formation constants of the complexes of Cu (ll), Co(ll), Ni (ll), Pb (ll) and Zn (ll) with these compounds have been determined at three different temperatures in 80% v/v water dioxane mixture by potentiometric measurements. The thermodynamic functions corresponding to stepwise complexation processes have been calculated at 30 0 C. The stepwise complexation is found to be predominantly as enthalpy driven process.