In a previous paper (Harris and Egerton 1938), the explosion of diethyl peroxide has been studied, and the limit of pressure at various temperatures at which the transition from unimolecular to explosive reaction takes place has been determined in various vessels. In the former work the influence of gases of high conductivity such as helium was not found to be as great in raising the critical pressure as might be expected, but apart from this it has been possible to fit both the older and a fresh series of results to the equations derived for a purely thermal explosion. The behaviour of the peroxide is compared with that of other compounds which show a similar transition to the explosive mode of decomposition, e.g. azomethane and ethyl azide.