A kinetic study has been made of the first-order thermal decomposition of the title compounds into ethvlene and the corresponding aza-substituted pyridines, between 650 and 713 K. The relative elimination rates at 650 K are (2-ethoxypyridine = 1): 0.545, 10.0, 1.03, 1.12, 9.68, and 3.28, respectively. The electronic effects of the aza ‘substituent’ are small, and a more important factor appears to be the C–N π-bond order; this latter accounts for the high reactivity of the pyridazines. The effects of the chloro substituent and of the aza ‘substituent’ are explicable in terms of a balance between electron withdrawal from the C–O bond (producing activation) and from the nitrogen involved in the cyclic transition state (producing deactivation). The effects of the chloro substituents confirm that the most important step of the reaction is breaking of the C–O bond. The statistically corrected rate (per ring nitrogen) of 2-ethoxypyrimidine is unexpectedly low. This may reflect difficulty in achieving the coplanar transition state in which the lone pairs in the s-orbitals of oxygen and the nitrogen not involved in the elimination are brought into close proximity.