The geometries and thermodynamic properties (DeltaH(f)(298), S-298, and C-p(T) (300-1500 K)) of 45 ketenes of four different types namely alkylketenes (set I), oxygenated alkylketenes (set II), 1,2- and 1,3-bisketenes (set III), and ketene-substituted alcohols (set IV) were determined by means of the CBS-Q procedure. The heats of formation of oxygenated ketenes were reevaluated using isodesmic reactions, and a systematic and consistent (similar to1.5 kcal/mol) over prediction in the stability of tert-butyl substituted systems has been identified at the CBS-Q level. Nearly 20 new thermochemical groups for the ketene molecular family and 3 nonnext neighbor interaction terms were evaluated from the computed thermodynamic functions by simultaneous parameter estimation. The nonnext neighbor corrections account for an eclipsing interaction between the >CCO group and its P bonds. Statistical analysis of the ab initio data demonstrates the validity of the group additivity approximation. The derived group values describe the ab initio DeltaH(f)(298), S-298, and C-p(300 K) with an average deviation of less than 0.06 kcal/mol, 0.15 cal/(mol K), and 0.18 cal/(mol K), respectively. In most cases, these quantum-calibrated group additivity estimates are expected to be more accurate than the limited experimental data and empirical estimates; the only significant discrepancy (similar to1.5 kcal/mol) is for substituted ketenes containing a tert-butyl group.