As usual for heterogenous reactions, the heterogeneous isotope exchange processes consist of several consecutive steps. The most important steps are the transport of the substances from the bulk to the interface, the exchange reaction, and then the reaction products leave the location of the exchange process and transfer into any phase. This latter process can imply dissolution, intraparticular, interparticular, sur­face diffusion, recrystallization, and so on. The overall kinetics of the isotope exchange process is determined by the slowest step(s); under steady-state condition, this is the rate-determining process.