Vibrational relaxation of the CH and CD (v = 2, 1) by the noble gases helium and argon has been monitored by laser-induced fluorescence following generation by pulsed laser photolysis of bromoform. Vibrational to translational relaxation is comparatively fast (k = 0.5-6.8 X 10(-14) cm(3) molecule(-1) s(-1)). Neither the isotopic dependence (k(CH) > k(CD)), or the variation with bath gas (k(AR) > k(He)), is compatible with the impulsive mechanism for V-T energy transfer developed by Schwartz, Slawsky and Herzfeld (SSH). A semi-quantitative explanation of the data has been developed based on the ab initio CH-Ar potential energy surfaces calculated by Alexander et al. [J. Chem, Phys. 101 (1994) 4547].