Diffusion and desorption are normally taken into consideration as the
main rate limiting processes for tritium release from solid breeder ma
terials in thermo-nuclear fusion devices. The importance of the correc
t identification of the process, in order to determine correct process
parameters, is universally recognized. Because the two processes are
temperature dependent, generally following an Arrhenius law, it is ver
y important to deter-mine, for a certain material microstructure, the
temperature ranges in which they are effective, or the existence of a
transition zone. The process identification in in-pile experiments is
normally very difficult and generally requires several measurements on
the same materials with different grain sizes. Even so it is almost i
mpossible to assure that materials, different in grain sizes, have ide
ntical all the other microstructural parameters. This again induces da
ta interpretation problems. In this paper the theoretical possibility
to discriminate between diffusion and desorption, especially for a par
ticular but interesting in-pile case, is pointed out. For the particul
ar case of big stepwise temperature decrease, the process, at the fina
l temperature, can be identified by a single step. The identification
is possible, in full analogy to the out-of-pile case, by observing the
slope of the early tritium release, at small times compared with the
residence time, in a log-log plot.