Methacrolein (MACR) plays an important role in atmospheric chemistry within the planetary boundary layer, as it is one of the major oxidation products of isoprene and has a short lifetime toward the hydroxyl radical (OH). In this study, quantum chemical techniques and statistical reaction rate theory have been used to simulate the addition of OH to MACR at conditions representative of the troposphere. In this chemically activated reaction, the time scales for product formation versus collisional deactivation of the vibrationally excited adduct are explicitly considered. Furthermore, the subsequent addition of O-2 is also incorporated within a single master equation, so as to investigate doubly activated peroxyl radical formation. The major reaction product of OH addition to MACR is the HOCH2C center dot(CH3)CHO radical formed via addition to the outer (beta) carbon. This radical is predominantly in the Z isomer although around a third of the population is quenched as the higher-energy E isomer. Calculated rate constants agree well with experiment when using M06-2X/aug-cc-pVTZ barrier heights, but are somewhat overpredicted using G3SX energies. The overall rate constant is controlled by competition between dissociation of the MACR center dot center dot center dot OH van der Waals complex back to reactants and isomerization on to MACR-OH adducts, which takes place on a time scale of several nanoseconds, but collisional deactivation of the MACR-OH adducts occurs on a time scale that is around an order of magnitude longer. When O-2 addition is included in the master equation, we observe that the MACR-OH adducts are removed by reaction with O-2 on a similar time scale to collisional deactivation. Around 50% of the subsequent peroxyl radical population is formed with some identifiable excess vibrational energy above singly activated [MACR-OH-O-2]*, with around 20% provided with an additional 20 kcal mol(-1) (>40 kcal mol(-1) relative to quenched MACR-OH-O-2) that can go into further unimolecular reaction. This double activation process is expected to lead to some prompt unimolecular decomposition of excited [MACR-OH-O-2]** peroxyl radicals to yield products including hydroxyacetone and methylglyoxal, regenerating the initiating OH radical in the process.