The kinetics of the reactions C2H3 + H-2 --> H + C2H4 (1) and CH3 + H-2 --> H + CH4 (2) have been studied in the temperature ranges 499-947 K (reaction 1) and 646-1104 K (reaction 2) and He densities (6-18) x 10(16) atoms cm(-3) by laser photolysis/photoionization mass spectrometry. Rate constants were determined in time-resolved experiments as a function of temperature. Ethylene was detected as a primary product of reaction 1. Within the above temperature ranges the experimental rate constants can be represented by Arrhenius expressions k(1) = (3.42 +/- 0.35) x 10(-12) exp(-(4179 +/- 67 K)/T) cm(3) molecule(-1) s(-1) and k(2) = (1.45 +/- 0.18) x 10(-11) exp(-(6810 +/- 102 K)/T) cm(3) molecule(-1) s(-1). Experimental values of k(2) are in agreement with the available literature data. The potential energy surface and properties of the transition state for reactions (1, -1) were studied by ab initio methods, Experimental and ab initio results of the current study were analyzed and used to create a transition state model of the reaction. The resulting model provides the temperature dependencies of the rate constants for both direct (1) and reverse (-1) reactions in the temperature range 200-3000 K : k(1) = 1.57 x 10(-20)T(2.56) exp(-(2529 K)/T) cm(3) molecule(-1) s(-1), k(-1) = 8.42 x 10(-17)T(1.93) exp(-(6518 K)/T) cm(3) molecule(-1) s(-1). Data on reactions 1 and -1 available in the literature are analyzed and compared with the results of the current study.