(Mn, Fe)2(P, Si)-type compounds are, to date, the most promising materials for refrigeration and energy conversion applications due to the combination of highly tunable giant magnetocaloric effect (GMCE) and low material cost.[1, 2] The GMCE of these compounds originates from the first-order magnetoelastic transition around the magnetic phase-transition temperature TC. However, the phase-transition temperature shows a peculiar thermal-history dependence in these compounds. As-prepared (Mn, Fe)2(P, Si) displays a significantly lower TC upon first cooling than on second and subsequent cooling processes. Since this behavior is only observed in as-prepared samples it is called the 'virgin effect'. The difference in TC between the first and second cooling processes of the as-prepared sample, hereafter referred to as ΔTC0, is taken as a measure of how strong the virgin effect is. The virgin effect is not exclusive to (Mn, Fe)2(P, Si) compounds being observed in other GMCE materials[3, 4], however its origin was for a long time unknown. In this study, we report our high-resolution neutron diffraction experiments that finally shed light on the origin of the virgin effect. Additionally, recovery of the virgin effect induced by thermal activation was observed experimentally.@en