A batch experiment was conducted with ten sludge treated soils wherein soils were equilibrated with various amounts of Cd-115 tagged Cd-fulvic acid (FA) for time intervals upto 72 h. The equilibrium models, including the Langmuir, two-surface Langmuir and Freundlich models and kinetic models including irreversible zero, first, second, two rate constant equation, second approximate and Elovich models, were evaluated for their ability to describe the retention of Cd-FA in sludge treated soils. The relationship between the amount of Cd-FA retained by soil and the concentration of Cd-FA in soil solution was described by either two-surface Langmuir or Freundlich models. The maximum sorption capacity of Cd-FA in soils at two sites of reaction obtained from the two-surface Langmuir adsorption isotherm equation was found to be positively correlated with the organic matter content and cation exchange capacity of the soils. In general, none of the equations describing the order of reaction could explain the data over the entire time and concentration ranges studied. The Elovich equation provided the best fit of the data indicating, thereby, that the sorption of Cd-FA might be due to complex reactions involving more than one site of reaction with different rates and energetics. The desorption of Cd-FA sequentially through 0.5 M Ca(NO3)(2) and 0.005 M KCl showed that a significant fraction of the Cd-FA complex retained by the soil was released to the solution but that some irreversible retention of Cd-FA occurred The possible mechanisms of retention of Cd-FA in soils are discussed.