Fluorescence from a single vibronic level of SO(B-3 Sigma(-), v' <= 3) prepared by a pulse excitation has been detected and thorough kinetic analyses for the photochemical processes have been made. Deconvolution analysis by the integrated-profiles method of the time-resolved fluorescence profiles recorded at various buffer gas (Ar and N-2) pressures has given the deactivation rates of initially prepared vibrational levels. Componential analysis of the dispersed fluorescence measured at different buffer gas pressures has provided the rate coefficients for level-to-level vibrational relaxation. It has been found that not only the single-quantum relaxation (Delta v = 1) but also the multiquantum relaxation (Delta v = 2 and 3) occurs by collisions with Ar and N-2. The efficiency of quenching is strongly dependent on the vibrational levels and correlates with the energies of the lowest nonfluorescent rotational levels. Candidates for the electronic states governing the quenching process have been discussed based on the kinetic and spectroscopic data.