Q. E. Unit of CTRL (ICAR) Department of Plant Breeding, Haryana Agricultural University, Hisar-125 004 D. N. (P. G.) College, Meerut Manuscript received 2 January 1989, revised 28 September 1989, accepted 26 July 1990 Acid dissociation constants of some substituted heterocyclic thioureas have been determined potentiometrically in aqueous ethanol (75% v/v) at different ionic strengths and thermodynamic dissociation constants have been calculated from these data. Ionisation constants at fixed ionic strength (0.1 mol dm-3) over the temperature range (10-50˚) and in different solvent-water systems have been determined. The thermodynamic parameters have been evaluated and effect of dielectric constants on dissociation constant have been discussed.