The influence of nitric oxide, propene, ethylene, toluene, and cyclohexene on the thermal decomposition of methylal and acetaldehyde dimethyl acetal in a static system has been studied. The reactions have been followed by observing the change of pressure with time and by infrared spectroscopy. Hydrogen cyanide, nitrous oxide, and probably a nitrite are produced from the nitric oxide, which is consumed during the transitory inhibition. The results obtained are discussed and it is shown that the normal methylal chain is several hundred steps long. A different behaviour is observed with unsaturated inhibitors. The pressure increase-time curves of the methylal decomposition are lowered by ethylene or propene below the limit reached with nitric oxide. Inhibition is not transitory. The amount of hydrogen in the products is preferentially diminished with any of either inhibitor. Ethylene or some of its products remaining adsorbed on the surface of the reaction vessel seem to have a specific influence on the decomposition of acetaldehyde dimethvl acetal.