The adsorption of humic acid and fulvic acid onto Bacillus subtilis cells and activated sludge biomass was studied as a function of pH and incubation time. The adsorption of humic and fulvic acids was strongly pH-dependent and followed the same trend on both surfaces, increasing in a sigmoidal way with decreasing pH over the 2-10 pH range. This behaviour is explained in terms of hydrophobic interactions between the uncharged biomass and the uncharged humic and fulvic acids. In contrast, the adsorption of phenol onto B. subtilis cells and activated sludge biomass showed in both cases an optimum pH at around 7.0. This optimum value may be interpreted in terms of a combination of hydrophobic interactions and hydrogen bonds between undissociated phenol and polar groups on the cell walls. Kinetic studies on the adsorption of humic acid, fulvic acid and phenol onto B. subtilis cells and sludge biomass pointed to a rapid uptake of the substances, with an equilibrium time of about 30 min. In all cases, the kinetic curves were acceptably fitted by non-linear regression to an exponential function, suggesting a first-order kinetic phenomenon. The specific adsorption values collected at optimum pH revealed that with the materials used in this work both B. subtilis and activated sludge follow the same adsorption trend: humic>fulvic>phenol. The lower adsorption of fulvic acid as compared with humic acid may be explained in terms of its lower hydrophobicity rather than its lower molecular size. On comparing the specific adsorption values of activated sludge versus B. subtilis, similar but lower figures were found for the three organic compounds studied. This similar behaviour suggests that both types of biomass base their adsorption capacity on the general characteristics of the bacterial cell wall, and the lower adsorption by the sludge would be due to a lower specific area due to clustering of the cells. This is remarkable, since sludge is a heterogeneous and cheap material in comparison with cultured bacterial cells.