In this paper, results are presented of the development of internal stresses in 2124 aluminium alloy reinforced with 25% of particulate silicon carbide. Measurements have been made during in situ low-cycle fatigue loading of this composite, using neutron diffraction. The neutron measurements were made using the ENGIN-X diffractometer at the ISIS neutron facility, UK. The results show how the internal stresses evolve with fatigue cycling. Eshelby-based modelling has been used to allow separation of the internal stress components in the MMC.