Methacrolein is a major product of isoprene ozonolysis, and methacrolein oxide is an important ozonolysis intermediate. We use CBS-QB3 and RRKM/master equation calculations to characterize all methacrolein formation pathways and all the unimolecular reactions of methacrolein oxide. Our predicted methacrolein yield agrees with experiment if we assume that all of the dioxirane formed from methacrolein oxide decomposes to methacrolein. The vinyl group of methacrolein oxide allows the species to cyclize to a dioxole with a reaction barrier lower than the barriers to either hydroperoxide or dioxirane formation. Two dioxole derivatives, 1,2-epoxy-2-methyl-3propanal and 2-methyl-3-oxopropanal, should be measurable products of isoprene ozonolysis. (C) 2007 Elsevier B.V. All rights reserved.