Gas phase (298.15 K, 101.325 kPa) adiabatic ionization energies (AIEs) were calculated for 236 organic compounds with the Gaussian-4 (G4) composite method and for 17 molecules at the W1BD level of theory. Functional group types considered span a range of mono- and polyfunctionalized halogenated, saturated and unsaturated, cyclic and acyclic, and heteroatom (N, O, S) substituted moieties without substantial conformational complexity. Excellent agreement was found using both computational methods against available experimental data. Approximately equivalent ATE prediction accuracy was observed between the G4 and the W1BD methods. For compounds with well-constrained experimental AIEs, both levels of theory provide effective chemical accuracy.