A method using isotopic dilution of carrier‐free sulfur (35S) for determining adsorbed sulfate (SO42‐) in soil samples from B horizons of podsolized soils was compared with an extraction of SO42‐ by 10 mM calcium dihydrogen phosphate [Ca(H2PO4)2]. The two methods gave comparable results for flesh soil samples while the extraction method resulted in significantly higher amounts of adsorbed SO42‐ when using air‐dried samples. Since the35S method assumes equilibrium between adsorbed SO42‐and SO42‐in solution, these results are evidence of complete exchangeability of adsorbed SO42‐extracted with Ca(H2PO4)2in fresh soil samples. It was thus concluded that the extraction method is a useful method when determining adsorbed SO42‐ in equilibrium with the soil solution in these soils. The35S reached equilibrium with the soil solution within 2 h. The difference between the methods when using air‐dried samples indicates that part of the adsorbed SO42‐ is irreversibly bound to the soil during air‐drying. Furthermore, it is recommended that the amount of SO42‐ determined in an extraction with water should be expressed as an intensity variable,i.e.,SO42‐ activity, rather than a capacity variable.