Aliphatic ketone anions (ketyl radicals) were produced by cocondensing aliphatic ketones and Na atoms in argon matrices at similar to 4 K and effecting electron transfer between them by mild radiation (lambda = 590 nm). ESR spectra of anion radicals of acetone, diethyl ketone, diisopropyl ketone, tert-butyl methyl ketone, and cyclobutanone were thus observed and analyzed. Examination of the acetone anion by ab initio SCF (UHF) and semiemperical (AM1) molecular orbital calculations yielded the structure and the conformation (of the methyl groups) in reasonable agreement with those deduced from the observed beta proton and alpha C-13 hyperfine coupling tensors. It is concluded that these anions have a semioccupied molecular orbital (SOMO) of the form R(2)C-O- (70%) <----> R(2)C(-)-O (30%) and are pyramidal in shape at the a carbon. In argon matrices at similar to 4 K, the beta alkyl groups are locked at a given azimuthal position, and the beta proton situated about the position trans to the unpaired electron orbital at the a carbon has a singularly large hyperfine coupling constant.