The microstructures of solvent-welded joints of irradiated poly(methyl methacrylate) (PMMA) have been investigated. The morphologies of tensile, tear, and cleavage surfaces were examined and the effects of the solubility parameter, welding temperature, and gamma -ray dosage on the mechanical strength were studied. Three solvents - methanol, ethanol, and N,N-dimethyl formamide (DMF) - were used with welding temperatures at 40, 50, and 60 degreesC. The gamma -ray dosages were 100, 200, and 300 kGy. The features of the tensile fracture surfaces exhibit either long and narrow hackles on the interface of PM1MA with low dosage, or lamellae in the adherent near the interface with high dosage for methanol and ethanol. For DME the morphologies of the tensile fracture surfaces at low and high dosages consist of many elliptical paraboloids and feathers, respectively. The morphology of the tear fracture surface of PMMA shows equiaxed hackles for methanol and ethanol and polygons for DME The cleavage surface appears to be chevron curves. The cleavage surface shows discontinuity of the chevron curves at the welded interface for non-irradiated specimens and continuity on irradiated specimens. The slope of the chevron curve at the interface is continuous for ethanol and DMF but is discontinuous for methanol. A solvent-affected zone was observed and its size had no correlation with the mechanical strength of the joined PMMA.