Department of Chemistry, Regional Engineering College, Durgapur-713 University Department of Chemical Technology, Bombay-400 019 Manuscript received 24 December 1982, revised 27 February 1984, accepted 3 May 1984 Precise calorimetric measurements of the CdII-thiourea and PbII-thiourea systems in 1 M NaCIO4 solution were made using a thermistor as the temperature measuring device in the system, and the values of the stepwise and overall changes in\(\bigtriangleup\)Go,\(\bigtriangleup\)Ho and \(\bigtriangleup\)So accompanying the complex formation reactions were evaluated. Values of the overall enthalpy (\(\bigtriangleup H^o_n\)) and entropy (\(\bigtriangleup S^o_n\)) changes for the mono-, bis-, tris- and tetrakis-coordinated species are for CdII ion -10.75 and -7.7, -34.04 and -63.5, -20.81 and -17.8, -43.35 and -66.5, respectively; while for PbII ion -12.25 and -28.4, -19.16 and -57.4, -10.61 and -19.4, -39.80 and -84.8, respectively at 30o with \(\mu\)= 1.0.