Primary dissociation pathways have been investigated for dimethyl ether by ab initio molecular orbital methods. Reactants, transition-state structures, and products were fully optimized up to the MP2/6-311G(2df,2p) level of theory. Relative energies have been calculated with the spin-projected PMP4 and CCSD(T) post-Hartree-Fock methods using the 6-311G(2df,2p) and the expanded 6-311++G(3df,3pd) basis sets. At the CCSD-(T)/6-3ll++G(3df,3dp) level, the barrier height for C-O bend fission is predicted to be 81.1 kcal/mol, while that for C-H bond fission is predicted to be 93.8 kcal/mol. These theoretical results agree very well with the experimentally measured barrier heights for these two bond fission channels. However, two new primary dissociation pathways are predicted to be competitive with these bond fission processes. These findings are discussed in light of previous experimental results on dimethyl ether.