A self-repairing polyester matrix composite material is described which utilizes embedded polyoxymethylene urea (PMU) microspheres to store a crack filling agent to be released into the crack and rebond the crack faces. The developed repair mechanism uses naturally occurring functional sites in the polyester matrix network to trigger the repair action. The repair agent is mostly composed of styrene monomers and high molecular weight polystyrene. Microscopic observations of the microsphere/crack interaction are presented. Of particular interest is the process with which the microspheres break and release their content when encountered by a crack. The effects of the microspheres on the stiffness and toughness of the polyester resin are also studied. Using standard tensile samples, the composite elastic modulus has been found to decrease with the volume fraction of microspheres, while the fracture toughness of tapered double cantilever beam (TDCB) specimens is shown to reach a maximum value at approximately 10% volume fraction. Examinations of the fracture surfaces show tails extending from the microspheres, indicating crack pinning and crack front bowing as the primary toughening mechanisms. An enhanced level of adhesion between the microsphere and matrix has been found to decrease the composite fracture toughness and increase the incidence of sphere breakage on the fracture surface.