Electron affinities and Delta H-acid are combined in a thermochemical cycle to arrive at bond dissociation energies for allene, methylacetylene, and the propargyl radical : D-0(CH2=C=CH-H) = 88.7 +/- 3 kcal mol(-1), D-0(H-CH2C=CH) = 90.3 +/- 3 kcal mol(-1), D-0(CH3C=C-H) = 130.2 +/- 3 kcal mol(-1), and D-0(CH2=C=C-H) = 100 +/- 5 kcal mol(-1). Electron affinity measurements were determined using negative ion photoelectron spectroscopy and yielded the following for the propargyl, 1-propynyl, and propadienylidene radicals : EA(CH2=C=CH) = 0.918 +/- 0.008 eV, EA(CH3C=C) = 2.718 +/- 0.008 eV, and EA(CH2=C=C) = 1.794 +/- 0.008 eV. Gas-phase acidity measurements were made using proton transfer kinetics in a flowing afterglow/selected-ion flow tube and yielded the following for allene, methylacetylene, and the propargyl radical : Delta G(acid)(CH2=C=CH-H) = 372.8 +/- 3 kcal mol(-1), Delta G(acid)(H-CH2C=CH) = 374.7 +/- 3 kcal mol(-1), Delta G(acid)(CH3C=C-H) = 373.4 +/- 2 kcal mol(-1), and Delta G(acid)(CH2=C=CH) = 364 +/- 5 kcal mol(-1). Delta G(acid) was converted to Delta H-acid by employing Delta S-acid : Delta H-acid(CH2=C=CH-H) = 381.1 +/- 3 kcal mol(-1), Delta H-acid(H-CH2C=CH)= 382.7 +/- 3 kcal mol(-1), Delta H-acid(CH3C=C-H) = 381.1 +/- 3 kcal mol(-1), and Delta H-acid(CH2=C=CH) = 372 +/- 5 kcal mol(-1). Evidence is provided for the isomerization of the allenyl anion (CH2=C=CH-) to the 1-propynyl anion (CH3C=C-) in the proton transfer reactions of CH2=C=CH- with CH3OH and CH3CH2OH. This complexity limits the precision of experimental measurements. This study explores the intricacies of determining gas phase acidity values by proton transfer reactions for systems in which isomerization can occur.