Studies of the unimolecular decomposition o f 4-methylpent-2-yne (M2P) and 4,4-dimethylpent-2-yne (DMZP) have been carried out over the temperature range of 903-1246 K using the technique of very-low pressure pyrolysis (VLPP). T h e primary reaction for both compounds is fission of the C-C bond adjacent to the acetylenic group producing the resonance-stabilized methyl-substituted propargyl radicals, CH:3C=CCH(CH3) from M2P and CH&=Cc(CH& from DM2P. RRKM calculations were performed in conjunction with both vibrational and hindered rotational models for the transition state. Employing the usual assumption of unit efficiency for gas-wall collisions, the results show that only the rotational model with a temperature-dependent hindrance parameter gives a proper fit to the V L P P data over the entire experimental temperature range. T h e high-pressure Arrhenius parameters a t 1100 K are given by the rate expressions log k 2 (sec-l) = (16. 2