The pyrolysis of acetaldoxime has been studied in the temperature range 330 to 440 $^\circ$C. The reaction is partly heterogeneous even in conditioned reaction vessels, and has an induction period. The orders of the decomposition with respect to oxime, and the activation energies, have been determined both for the reaction occurring during the induction period and for the fast subsequent reaction. The effect of several additives has been studied. Propylene acts as an inhibitor, but nitric oxide causes a considerable acceleration. Analytical results for the nature and relative importance of the products are given. A mechanism is suggested which postulates that the reaction is predominantly free radical in nature, but may proceed to some extent through a molecular mechanism. The reaction during the induction period, it is suggested, leads mainly to the formation of methyl cyanide and water, but a concurrent heterogeneous oxidation to acetaldehyde and nitrous acid occurs to a small extent. The rate of build up of the latter determines the length of the induction period. The fast reaction is a consequence of the break down of nitrous acid giving rise to catalytic chain mechanisms involving nitric oxide and oxygen.