The toughening of thermosets through the creation of residual compressive stresses around microspheres was studied. Expandable, hollow microspheres containing gas with heating were used for the creation of residual compressive stresses. Microscopic compressive residual stresses in the vicinity of the crack tip were graphically analyzed and related to the macroscopic mechanical behavior for mode I fracture. The graphical analysis and experimental results confirmed that toughening was achieved mainly because of residual compressive stresses rather than a postcure effect. As expected, however, no toughening effect was found in mode III fracture. (c) 2006 Wiley Periodicals, Inc.