The isotopic exchange rate of Cs+between hydrous niobium(V) oxide in the Cs+form and aqueous solutions was determined radiochemically. The rate was controlled by the diffusion of Cs+in the exchanger particle at varying pH. The diffusion coefficients at 10°C increased with pH up to pH7; from 3.0x10−11m2s−1at pH6 to 4.0×10−11m2s−1at pH7, and became constant above pH8 (5.0x10−11m2s−1). While the diffusion coefficients of Na+monotonously decrease with increasing pH; from 7.9×l0−11m2s−1at pH6.5 to 2.8×l0−11m2s−1at pHll. The difference in the dependence of diffusion coefficients on pH between Cs+and Na+was interpreted in terms of strength of interaction between counterions and ion-exchange sites since hydrous niobium(V) oxide has selectivity higher for Cs+than for Na+.