Infrared chemiluminescence from HF and HCl has been observed to yield vibrational and rotational population distributions from the reactions F + H 2,H 2S, CH 3SH and Cl + H 2S, CH 3SH. An apparatus giving nearly arrested relaxation distributions is described. The cold wall reaction vessel is optically coupled with a commercial Fourier transform spectrometer. The following relative populations were determined: N υ=1: N υ=2: N υ=3 = 0.22:0.52:0.26 for F + H 2; N υ=1: N υ=2: N gn=3: N υ=4 = 0.26:0.33:0.27:0.14 for F + H 2S; N υ=1: N υ=2: N υ=3: N υ=4 = 0.31:0.35:0.30:0.04 for F + CH 3SH; N υ=1: N υ2 = 0.90:0.10 for Cl + CH 3SH. In the series F + H 2, H 2S, and CH 3SH the energy released as HF vibrational excitation ƒ υ = 0.64, 0.56 and 0.45, respectively) decreases with a reduction of the bond energy D 0 0(R-H) in the hydride. The bond energies D 0 0 (H-SH) ⩽ 388 kJ/mol and D 0 0 (CH 3S-H) ⩽ 366 kJ/mol follow from the HCl emission of the reactions Cl + H 2S - HCl(υ ⩽ 1) + SH and Cl + CH 3SH - HCl(υ ⩽ 2) + CH 3S(CH 2SH). The first value is consistent with the results of the F + H 2S studies. For F + CH 3SH, however, the energy corresponding to the highest observed HF (υ, J) levels is 12 kJ/mol less than the thermochemical limit set by the Cl + CH 3SH reaction. Evidence for both a CH 3S and a CH 2SH formation pathway exists.