The predissociation mechanism of the (C) over tilde state of SO2 have been investigated by the measurements of degenerate four-wave mixing (DFWM), laser-induced fluorescence (LIF) and photofragment yield spectra. The DFWM spectrum of jet-cooled SO2 was measured under a high power condition where the DFWM signal corresponds well to the absorption spectrum. Relative fluorescence quantum yields of various vibronic bands were obtained by comparing the DFWM spectrum with the I-IF spectrum, It was found that the fluorescence quantum yields of the vibronic levels involving anti-symmetric stretching (v(3)) vibration are considerably smaller than those of other levels, The result indicates that the nonradiative process of the C state is initiated as the internal conversion to the (X) over tilde state and v(3) acts as the promoting mode, By observing the O atom photofragment yield spectrum we confirmed that the predissociation occurs from the (1,4,2) level. Vibronic dependence of the rise time of the O atom generation was found to be in accordance with those of the fluorescence decay rate, The obtained results represent that the dissociation occurs immediately after the internal conversion to the X state. (C) 1997 American Institute of Physics.