The potential energy surfaces of acetyl cyanide and its functional isomer acetyl isocyanide in their electronic ground state have been investigated using ab initio molecular orbital MP2(FU)/6-311G** and QCISD(T)/6-311G** calculations. The molecular eliminations of CH3COX (X = CN,NC) to CH3X + CO, HX + CH2CO, and CH2 + HCOX, the unimolecular rearrangements to CH3C-OX and CH3C - CX carbenes, 1,3-hydrogen migration to CH2 = C(OH)X, 1,3-methyl migration to CH3NCCO, and radical decompositions to X. + CH3CO and CH3. + COX have been examined. Also the secondary decomposition processes of COCN and CONC radicals in their ground states and the secondary elimination of HX from CH2 = C(OH)X have been investigated. Cyanide-isocyanide isomerization is found to be the predominant channel in the unimolecular reactions of acetyl cyanide. Despite being exothermic, the decomposition of acetyl cyanide into CH3CN + CO is found to be kinetically unfavorable. Decomposition into HCN + CH2CO, the reverse reaction of CH3COCN synthesis, is found to be a preferred dissociation pathway and is expected to compete at high temperatures with the 1,3-hydrogen migration, yielding cyanovinyl alcohol. The barriers for the rearrangement processes to carbenes in acetyl isocyanide are quite high in contrast to those of acetaldehyde and fluoroformaldehyde. The large magnitude of the reverse barrier from the carbenes to acetyl cyanide suggests the stability of the carbenes. In the ground state of CH3COX, the CH3-C bond cleavage is found to be more facile compared to the C(O)-X bond cleavage. The secondary dissociation from CONC requires a rather small barrier, and both cyanide and isocyanide forms are found to have very similar potential energy surface.