The thermal isomerization of cyclopropene in the vapor phase in the temperature range from 472 to 502 °K led to 1-propyne as the only product. At pressures of cyclopropene of 1-3 Torr, and in the presence of an inert gas (carbon dioxide) pressure of 50-60 Torr, the kinetics of the decomposition was homogenous and obeyed the first-order rate equation up to 88% conversion. From the temperature dependence of the reaction at total pressures of ∼60 Torr, the first-order rate constant was observed to fit the equation: k = 1012.13 e-35200 ± 1300/RT sec-1. In the absence of a considerable excess of diluent gas, the reaction was not “clean”: the kinetics did not follow the first-order rate equation and the rate was greatly influenced by an increase in surface. The thermal decomposition of 1-methylcyclopropene was also studied at reactant pressures of 1-3 Torr and inert gas (carbon dioxide) pressures of 50-60 Torr. In the temperature range from 474 to 499 °K the kinetics of the over-all reaction fitted a first-order rate equation, the rate constant being given by the expression: k = l011.4e-34700±1200/RT sec-1. The products that were observed were (in the order of decreasing importance) 2-butyne, 1,3-butadiene, and 1,2-butadiene. © 1969, American Chemical Society. All rights reserved.