Rate constants were determined for the reactions of CH((XII)-I-2,v=0) with H-2 and D-2 and for the relaxation of CH((XII)-I-2,v=1) by Hz and D-2. The method, employing pulsed laser photolysis to generate CH radicals and laser-induced fluorescence to observe their rate of removal, was implemented between 744 and 86 K in heated and cryogenically cooled cells and from 295; to 13 K in a Cinetique de Reaction en Ecoulement Supersonique Uniforme (CRESU) apparatus, The rate constants for the reaction of CH(v=0) with D-2 Were determined from 13 to 584 K and those for the removal of CH(v=1) by H-2 and D-2 from 23 to 584 K. These rate constants show no dependence on total pressure and a mild negative temperature dependence, and they are clearly related closely to the rate of capture to form a strongly bound CH3* or CHD2* energized collision complex. The rate constants for the reaction of CH(v=0) with H-2 were measured from 53 to 744 K. By contrast, their values depend in a complex fashion on temperature and total pressure, the latter effect having been studied from 4 to 400 Torr in heated and cooled cells and from 0.28 to 4.5 Torr al 53 K in the CRESU apparatus. It is clear that reaction proceeds via a CH3* complex which, favored by high temperatures and low total pressures, can dissociate to give CH2+H, CH((XII)-I-2)+H-2-->CH2+H; Delta(r)H(298)(0)=+(12.0+/-5.2) kJ mol(-1) or, especially at low temperatures and high total pressures, may be stabilized by collisions to yield CH3 radicals CH((XII)-I-2)+H-2+M-->CH3+M; Delta(r)H(298)(0)=-(450.(7)+/-2.0) kJ mol(-1). Careful analysis of the pressure-dependent fall-off behavior yields the following expressions for the rate constants (in the case of the association reaction, in the limit of low pressure) : k((1a))=(3.1+/-0.7)x10(-10) exp(-(1650+/-100)/T) cm(-3) molecule(-1)s(-1), k((1b))(0)[Ar]-(5.2+/-1.5)x10(-30)(T/298)(-1.6+/-0.22)[Ar] cm(3) molecule(-1)s(-1). Comparison of the rate data obtained for the reaction of CH(v=0) with D-2 and for the removal of CH(v=1) by H-2 and D-2 is consistent with all three processes occurring through the formation of a CH3* (CHD2*) complex whose rate of formation is independent of the vibrational state of the CH. Consequently the rate constant for the reaction in the limit of high pressure can be estimated from that for the removal of CH(v=1) by H-2 yielding : k((1b))(infinity)=(1.60+/-0.11)x10(-10)(T/298)(0.08+/-0.05) cm(3) molecule(-1)s(-1).