Experimental studies have been conducted to determine the rate coefficient and mechanism of the reaction between methylglyoxal (CH₃COCHO, MGLY) and the OH radical over a wide range of temperatures (233–500 K) and pressures (5–300 Torr). The rate coefficient is pressure independent with the following temperature dependence: k₃(T) = (1.83 ± 0.48) × 10^–12 exp((560 ± 70)/T) cm³ molecule^–1 s^–1 (95% uncertainties). Addition of O₂ to the system leads to recycling of OH. The mechanism was investigated by varying the experimental conditions ([O₂], [MGLY], temperature and pressure), and by modelling based on a G3X potential energy surface, rovibrational prior distribution calculations and master equation RRKM calculations. The mechanism can be described as follows: Addition of oxygen to the system shows that process (4) is fast and that CH₃COCO completely dissociates. The acetyl radical formed from reaction (4) reacts with oxygen to regenerate OH radicals (5a). However, a significant fraction of acetyl radical formed by reaction (R4) is sufficiently energised to dissociate further to CH₃ + CO (R4b). Little or no pressure quenching of reaction (R4b) was observed. The rate coefficient for OD + MGLY was measured as k₉(T) = (9.4 ± 2.4) × 10^–13 exp((780 ± 70)/T) cm³ molecule^–1 s^–1 over the temperature range 233–500 K. The reaction shows a noticeable inverse (k_H/k_D < 1) kinetic isotope effect below room temperature and a slight normal kinetic isotope effect (k_H/k_D > 1) at high temperature. The potential atmospheric implications of this work are discussed.