In situ observations of the resident concentration and penetration of reactive chemical underneath a disc permeameter are used to infer the adsorption isotherm that characterizes soil-solution exchange in two structured soils. In permeameters set to the unsaturated head of -40 mm, the use of the dual tracers of inert bromide and reactive 35S, in two contrasting soils, enabled resolution of their sulfate adsorption isotherms. In Manawatu fine sandy loam, a mobile-immobile water soil, measurements of the resident concentration of 35S and Br- under the disc showed sulfate to be only weakly adsorbed. However, an analytical solution for the one-dimensional dispersive entry of reactive chemical into soil indicated that for any degree of adsorption, substantial depths of infiltration (≫200--300 mm) would be required for the adsorbed concentration (S, mol kg-1) to reach equilibrium with the concentration of the influent solution (c, mol L-1). But this same equation showed that under the disc, the retardation of the reactive chemical (35S) behind the inert tracer (Br-) could be used to infer the strength of the adsorption exchange. When carried out at two different concentrations, this retardation procedure can be used to infer the nonlinear Freundlich isotherm of exchange. In the Ramiha silt loam, a fully mobile water soil, different retardations were observed during concurrent unsaturated infiltration of Br- and labeled sulfate at 20 &mgr;mol L-1 and at 2 mmol L-1 SO2-4. The isotherm was found to be S=2.3c0.7. This is less than the S=3.0c0.53 measured by the standard batch procedure on saturated 1:5 soil solution samples. ¿ American Geophysical Union 1996