Department of Chemistry, Inorganic Chemistry Division, University College of Science, 92, Acharya Prafulla Chandra Road, Calcutta-700 009. (India) Manuscript received 10 May 1978, accepted 7 July 1978 Dissociation of tris-biguanide complexes of Cr(III) and Co(III) catalysed by some bivalent metal ions in acid medium has been studied. Linearity of ∆H# vs ∆S# corresponding to km for the different catalysing metal ions and each of the substrate complexes as well as of km for Co(III) complex vs km for Cr(III) complex for the different catalysing metal ions indicate similar mechanism in all the cases. The catalytic activity of the different metal ions decreases in the sequence Cu(II)>Ni(II)> Co(II)> Zn(Il)>Mn(II), with log km varying linearly with Z*/(rM2 +)2 of the catalysing metal, i.e., its complexing ability. From the results it has been possible to evaluate the values of the formation constants of the monobiguanide complexes of the other catalysing metal ions from a knowledge of the corresponding value of the Cu(II) complex and the km values.