In this work it is shown how the microscopic residual stress state in 6061Al-15vol%SiCw composites can be modified by plastic strain and by isothermal annealing (the latter, affecting also the precipitation state). The composites were obtained by a powder metallurgical route, which involves extrusion of Al-SiCw powder blends. Neutron diffraction measurements were carried out to determine the RS state. Several degrees of plastic strain, from 1% up to 15% in compression, have been applied ex-situ to the same specimen on several composite materials. Furthermore, the m-RS was also investigated in one of the composites at several precipitation states as resulting from isothermal annealing at 300°C during different times. Investigations were carried out from a fully hardened state, T6 up to an overannealed condition, OV (obtained after annealing during 100 h). The results show that quenching from solid solution treatment temperature prior to annealing at 146°C to achieve the T6 condition provokes a RS state, which does not relax during this treatment. No decrease of the deviatoric component and partial relaxation of the hydrostatic term of the micro- RS developed in the composite occur when treating from the T6 to the OV condition. Instead, only 1% plastic strain is sufficient to produce total relaxation of the hydrostatic term of the m-RS. Even inversion of the radial component occurs on higher straining. Again, no relaxation of the deviatoric component occurs. This may be explained bearing in mind that the deviatoric component is linked to the geometry of the reinforcing particles (whiskers degree of orientation). The hydrostatic term does not relax totally after heat treatment because is partially re-generated on cooling, while the plastic deformation is ‘locked’ in the microstructure via dislocation activity.