The ability of acetylenic systems to participate in intramolecular reactions proceeding via 6-membered cyclic transition states was established by thermolysis of substituted Beta-hydroxyacetylenes, in both the vapor and the liquid states. Thermolysis products consisted solely of those allenes and carbonyl compds. derivable from a 1,5-H transfer. The homogeneous reaction followed the 1st-order rate law. Activation energies with a variety of alkyl substituents were within experimental error. The activation parameters, Ea~40kcal/mole and activation entropy ~-10 entropy units, indicate a cyclic transition state and closely parallel the parameters for the cleavages of analogous olefins. Acetylenes utilized reacted at rates varying from 1.3 to 7 times as fast as those of their olefinic analogs. The effects of alkyl, pH, and vinyl substituents, and relative rate comparisons with the reaction of analogous olefins, are in accord with a planar transition state for the participation of the triple bond. Chem. Abstr., 76, 33516v, 1972.