In order to better understand the fate of Cd in soils it is important to ascertain the influence of soil humic substances on the retention of Cd in soils. In this study the acid-base and Cd complexation properties of a humic acid fraction extracted from a soil were investigated using potentiometric titrations. A number of electrostatic and nonelectrostatic equilibrium models were considered in order to obtain the simplest model that would best describe the titration data. All proton binding and Cd complexation constants were determined using the optimization procedure in the computer program FITEQL. The model that best fit the experimental data was a nonelectrostatic model, which included two diprotic acids, H2 A and H2B. The dissociation constants for H2A were calculated to be log β-1,0,1,0 = −4.00 ± 0.02 and log β-2,0,1,0 = −9.32 ± 0.03 and for H2B were calculated to be log β-1,0,0,1 = −7.43 ± 0.24 and log β-2,0,0,1 = −16.66 ± 0.31. The model indicated the presence of four Cd complexes, CdHA+, CdA0, CdH-1A− and CdHB+, with complexation constants calculated to be log β-1,1,1,0 = −1.29 ± 0.09, log β-2,1,1,0 = −5.92 ± 0.07, log β-3,1,1,0 = −14.39 ± 0.09, and log β-1,1,0,1 = −3.72 ± 0.18.