The effects of size and geochemical properties on the binding of trace metals to natural colloids and particles have been investigated. Suspended particulate matter (SPM) from the River Mersey in NW England was fractionated by centrifugation to give three size fractions (nominally 0.05–0.5 μm, 0.5–1.0 μm and >1.0 μm). The SPM was characterized by scanning electron microscopy and by carbon and nitrogen analysis. Large proportions of the particles were microbial in origin, dominated by diatoms in the largest size fraction and bacteria in all fractions. Acid-base titrations indicated a significant difference between the proton binding characteristics of the three samples. The smallest fraction had the greatest charge per unit mass whereas the largest fraction had the least charge: 2.0 and 1.0 meq g−1 charge developed between pH 4 and 10, respectively. Experimental sorption studies with Cd and Cu indicated that metal binding per unit mass of SPM varied little between the three size fractions, although Cd was more strongly bound to the two smallest fractions. A simple one-site binding model provided a good description of the data and showed that the observed Cd and Cu sorption constants were consistent with literature values. The findings indicate that metal binding to the three size fractions is controlled mainly by the mass concentration and pH. The dependence on mass suggests that the surface area effective for binding is substantially independent of the size class. The results question the importance of the role played by the sub-micron fraction in trace metal binding by natural particle assemblages.