Adsorption of the naturally occurring pyrimidine nucleobases and their corresponding nucleosides and nucleotides on compost-derived humic acid (HA) has been studied in aqueous solution. All these nine solutes interact with HA. Adsorption isotherms were measured at 19 solute concentrations from 12-600 mu M and 6 temperatures in the range 10.0-40.0 degrees C. Analysis with the Langmuir model indicates that adsorption occurs in three detectable steps A, B, and C, depending on the solute and the experimental conditions. Most of the data fit the Langmuir model, which generates a site capacity vi and an equilibrium constant K-i for each detectable adsorption step. Site capacity and solute selectivity data indicate that the HA sites responsible for adsorption steps A, B, and C are chemically different and that pyrimidine solutes are adsorbed through their nucleobase units. Adsorption of pyrimidines on HA is exothermic, thermoneutral, or endothermic, depending on the solute and the adsorption step. Linear correlation of adsorption enthalpies and entropies points to similar surface interactions.