This paper primarily describes thermomechanical procedures for improving the mechanical properties of fibre composites. These procedures are based on the principle of introducing internal stresses into the laminate by permanent deformation of the matrix. These internal stresses are intended to reduce stress concentrations within the matrix of the loaded composite and thus result in an increase of strength. Basically two different procedures were used. In the first one, completely cured laminates were heated to temperatures above the region of the glass transition temperature, then loaded and finally cooled down under load to room temperature. In the second procedure, laminates not completely cured were heated to curing or post-curing temperatures or both, then loaded and stored under load until being completely cured. With both procedures permanent deformations within the matrix were achieved. These deformations resulted in a substantial increase in elongation upon the onset of typical damage development under tension loading, and to some extent in an increase in compression strength as well as an improvement of fatigue behavior.