The systematics of isotopic exchange in a heterogeneous batch system between a dilute aqueous solution and an adsorbate or the solid adsorbent are discussed. In particular, the influence of the experimental precision on the detection of an eventual non-reactive fraction of the adsorbate is considered, down to ~ 5% of which is detectable. The exchange with the solid adsorbent is illustrated by the case of45Ca2+ and calcite. The minimum layer thickness observable in surface exchange is ~ 0.05 m. The same formalism can be applied to uptake due to recrystallization of the solid. It enables distinction between this phenomenon and diffusion into the solid.