Abstract: The vibrational relaxation rates of PbS~*A(0~+) state in Ar and S_2 have been studied by laser-induced fluorescence technique. The vibrational relaxation rate of PbS~*A(0~+) in S_2 is two orders of magnitude larger than in Ar. Formation of unstable intermediate PbS_3 may be responsible for the fast relaxation. The energy transfer process between A(0~+)(V'=19) and B(1)(V'=6) states is also suggested.