A fundamental study of the application of humic acid (HA) to the recovery of cadmium and lead from dilute solutions was carried out. HA, which has an excellent complexing ability with heavy metals, was insolubilized by heating at 330 degrees C for 1 h and was used as an adsorbent. A metal-complexation model was used to compare the adsorption characteristics of insolubilized humic acid (IHA) with those of HA. The results showed that two different types of acidic groups were present on HA and the insolubilization process scarcely influenced the metal-complexation constant of acidic groups; however, the number of available metal binding sites on IHA was decreased to some extent.