A comparative investigation of the complexations of divalent metal ions (Cd, Cu, and Pb) by a well characterized soil humic acid (HA) from Okchun Metamorphic Belt was carried out in 0.05 M $KNO_3$ and pH of 4.5 using ion selective electrodes. A continuous distribution model based on the Scatchard Plot was used to determine the stability constants, because the constants obtained by this modeling technique takes the variations in binding energies into consideration without regards to the manner in which M(II) ion is bound on HA. The mean value of log $K_i$ were $4.05{pm}0.60,;4.92{pm}0.36,;and;5.63{pm}0.34;{ell};mol^{-1}$ for Cd(II)-, Pb(II), and Cu(II)-humate complexes respectively. The values of intrinsic constant (log $K_{int}$; binding at strongest site) were $7.12{pm}0.30,;6.59{pm}0.32,;and5.07{pm}0.56;{ell}; mol^{-1}$ in the order Cu(II) > Pb(II) > Cd(II) ion.