The lattice strain induced by the thermal evolution of B–B pairs formed in a Si 1− x B x/Si layer as a consequence of He irradiation has been studied in situ in an N 2 atmosphere, by using a high resolution x-ray diffractometer equipped with a hot stage sample holder. The collection of repeated rocking curves during a linear temperature (T) ramp allowed monitoring of the effects of the B–B pair thermal evolution on the epilayer lattice parameter a (and equally its strain) during the whole of the annealing from room T up to their complete dissolution (883 C). By analysing the evolution of a (T) we extracted detailed information about the kinetics of B–B pair evolution. This allowed us to determine an experimental description of the B–B pair dissolution path in good agreement with recent ab initio calculations.