The synthesis and labeling (with S-35) of a water-soluble random copolymer of styrenesulfonate and tert-butylstyrene (20 mol %) is described. Adsorption of this copolymer to the toluene/water interface is followed in situ using a proximity radiometric detection scheme, where the organic phase also serves as a scintillator. Adsorption isotherms, determined with the aqueous phase either salt-free or containing 1 M NaCl, are of the high-affinity type, with respective pseudolimiting adsorbed amounts of 0.24 and 1.4 mg m(-2). Adsorption is fairly rapid, occurring with a time constant of a few minutes. Self-exchange of radiolabeled polymer at the interface, performed with a solution excess of unlabeled polymer, is complete within a few hours, providing evidence that the surface excess is under thermodynamic, rather than kinetic, control. Preliminary comparisons of the liquid/liquid interface and a chemically similar polystyrene/water interface reveal that the adsorption isotherms are similar despite the additional mobility afforded to adsorbed segments at the fluid interface. The free energy of adsorption for tert-butylstyrene units is estimated to be ca. 13 kT from solubility and partitioning measurements of the corresponding monomer.