A standard analytical technique for determination of the partial ion exchange capacity of mineral or soil for selective uptake of cesium, the specific radiocesium interception potential (SRIP) is formulated and theoretically discussed. The method is based on the determination of a retained, leached or sorbed fraction of carrier-free cesium-137 in soil contacted with 0.01M KNO3-0.01M Ag thiourea complex solution at the phase ratio solution:sorbent 10 ml:0.1 g. Reliability of the method is discussed in connection of radiocesium carrier and humic substances presence.