Evidence has been collected which shows that knock in an internal combustion engine cylinder is intimately connected with the presence of peroxidic substances of characteristic structure formed in the last part of the charge to burn. Such peroxides might be formed either in the primary oxidation of the hydrocarbon, or from aldehydes which are known to be an intermediate product, The experiments described, in the paper cited, left somewhat undecided the question whether or not peroxides arising from aldehydes formed during combustion are responsible for engine knock, although there was evidence against such a conclusion. It was also left uncertain to what extent aldehydes other than formaldehyde were present in the gases sampled from the engine cylinder, The purpose of this note is to describe some further experiments on these questions, which make it very unlikely that aldehyde peroxides are those that are mainly responsible for knock; this conclusion strengthens the evidence for the primary formation of peroxides in the oxidation of the higher hydrocarbons. First, experiments on various fuels show that aldehydes other than formaldehyde are formed in amounts inadequate to account for the knock observed; and formaldehyde is shown not to cause knock even when its concentration is much greater than ever arises from the combustion of fuels which knock. Furthermore, there is no correlation between the amount of higher aldehydes formed by any fuel and its tendency to knock. Secondly, experiments were made to test whether fuels which do not readily knock fail to do so on account of their not forming sufficient primary peroxide of suitable structure, or because of a radically different mechanism of combustion, The results suggest that the former is the reason; for instance, amylene containing appreciable amounts of olefine peroxide (p. 112) did not knock, whereas an addition to it of small quantities of amyl nitrite had a pro-knock effect even more pronounced than its effect on a fuel such as petrol.