The gas phase pyrolyses of crotonic and acrylic-trifluoroacetic anhydrides were shown to yield ethylidene ketene [CH3CH= C= C= O] and methylene ketene [CH2= C= C= O], respectively. The former was identified via the collisional activation mass spectrum of its molecular ion. The ionization energies of the two ketenes, 8.68 and 9.12±0.05 eV respectively, measured using energy selected electrons, lead to 215 and 233 kcal mol-1 ...