A series of eight thermal cheletropic decarbonylations show dramatic differences in reaction pathways and in activation energies depending on the molecular orbital topology, as calculated by using ab initio molecular orbital theory (MP2(FC)/6-31G* optimized geometries and MP4/D95** + ZPE single point energies). The decarbonylations of 3-cyclopentenone (1) and bicyclo[2.2.1]hepta-2,5-diene-7-one (3) are pericyclic, orbital symmetry allowed reactions, but it is argued that the decarbonylation of cyclopropanone (9), although formally orbital symmetry allowed, lacks an energy of concert and thus is 'effectively forbidden'. The carbon monoxide produced from 1 is predicted to be formed vibrationally cool and rotationally hot. Fragmentations of 2,3-furandione (5) and 2,3-pyrroledione (7) are pseudopericyclic reactions with two orbital disconnections, proceed via planar transition structures, and have activation energies that are much lower than expected for pericyclic reactions of comparable exothermicity. It will be an experimental challenge to determine if the carbon monoxide product from each of these is formed with little vibrational or rotational excitation as predicted. Fragmentations of 3H-furan-2-one (11), 3-cyclopentene-1,2- dione (13), and 3-methylene-3H-furan-2-one (15) each have a single disconnection. Strong bonding at the orbital disconnection in the transition structure tends to lower the barrier and give the reaction more pseudopericyclic character.