The mass-resolved anionic products of the reaction of O center dot- with acetaldehyde, H3CCHO, are studied using photoelectron imaging. The primary anionic products are vinoxide, H2CCHO-, formylmethylene anion, HCCHO center dot-, and ketenylidene anion, CCO center dot- From photoelectron spectra of HCCHO center dot-, the electron affinity,of triplet (ground state) formylmethylene (1.87 +/- 0.02 eV) and the vertical detachment energy corresponding to the first excited triplet state (3.05 eV) are determined, but no unambiguous assignment for singlet HCCHO could be made. The elusive singlet is a key intermediate in the Wolff rearrangement, resulting in formation of ketene. The fast rearrangement associated with a large geometry change upon photodetachment to the singlet surface may be responsible for the low intensity of the singlet compared to the triplet bands in the photoelectron spectrum. The title reaction also yields CCO center dot-, whose formation from acetaldehyde is novel and intriguing, since it requires a multistep net-H-4(+) abstraction. A possible mechanism is proposed, involving an [H2CCO center dot-]* intermediate. From the measured electron affinities of HCCHO (above), H2CCHO (1.82 +/- 0.01 eV), and CCO (2.31 +/- 0.01 eV), several new thermochemical properties are determined, including the C-H bond dissociation energies and heats of formation of several organic molecules and/or their anions. Overall, the reactivity of O center dot- with organic molecules demonstrates the utility of this anion in the formation of a variety of reactive intermediates via a single process.