[en] Neutron diffraction has been used to measure macro- and microstress components as functions of position within tension and compression samples of an Al-Si-Mg composite. The influences on these components of both tensile and compressive plastic deformation have been studied. Deformation to about 1.5% plastic strain (for both tension and compression) relieves the macrostress and most of the thermal microstress but sets up new residual microstresses. The results for the microstresses measured for the silicon phase following plastic deformation and unloading, are compared with the predictions of a finite-element model for the micromechanical behaviour of the alloy.