Tension tests were conducted in air at room temperature on PMMA sheet specimens which had been previously soaked in a 40 vol% acetonitrile aqueous solution at 20 degrees C for 24 h and then dried in air at room temperature for 480 h. In contrast with an untreated specimen which fractured at a stress of 84 MPa and a strain of 9%, shear yielding clearly took place at 42 MPa and the elongational fracture strain increased to about 148%. No crazes were observed on the specimen surface and as a result the transparency of the PMMA was thoroughly maintained until fracture. Thus this soaking treatment may change PMMA to a completely ductile polymer without a crazing mechanism. The results of the dynamic viscoelastic measurements at 1 Hz show that the glass transition temperature was lowered to about 80 degrees C (as compared to about 110 degrees C), and the beta relaxation became much sharper with a higher peak value of 20 degrees C (as compared to a broad curve with a peak at 50 degrees C). This clear beta relaxation at room temperature may contribute to shear yielding and large plastic elongation of the treated PMMA.