Decomposition kinetics of stabilised CH₂OO and CD₂OO Criegee intermediates have been investigated as a function of temperature (450–650 K) and pressure (2–350 Torr) using flash photolysis coupled with time-resolved cavity-enhanced broadband UV absorption spectroscopy. Decomposition of CD₂OO was observed to be faster than CH₂OO under equivalent conditions. Production of OH radicals following CH₂OO decomposition was also monitored using flash photolysis with laser-induced fluorescence (LIF), with results indicating direct production of OH in the v = 0 and v = 1 states in low yields. Master equation calculations performed using the Master Equation Solver for Multi-Energy well Reactions (MESMER) enabled fitting of the barriers for the decomposition of CH₂OO and CD₂OO to the experimental data. Parameterisations of the decomposition rate coefficients, calculated by MESMER, are provided for use in atmospheric models and implications of the results are discussed. For CH₂OO, the MESMER fits require an increase in the calculated barrier height from 78.2 kJ mol⁻¹ to 81.8 kJ mol⁻¹ using a temperature-dependent exponential down model for collisional energy transfer with 〈ΔE〉_down = 32.6(T/298 K)^1.7 cm⁻¹ in He. The low- and high-pressure limit rate coefficients are k_1,0 = 3.2 × 10⁻⁴(T/298)^−5.81exp(−12 770/T) cm³ s⁻¹ and k_1,∞ = 1.4 × 10¹³(T/298)^0.06exp(−10 010/T) s⁻¹, with median uncertainty of ∼12% over the range of experimental conditions used here. Extrapolation to atmospheric conditions yields k₁(298 K, 760 Torr) = 1.1^+1.5_−1.1 × 10⁻³ s⁻¹. For CD₂OO, MESMER calculations result in 〈ΔE〉_down = 39.6(T/298 K)^1.3 cm⁻¹ in He and a small decrease in the calculated barrier to decomposition from 81.0 kJ mol⁻¹ to 80.1 kJ mol⁻¹. The fitted rate coefficients for CD₂OO are k_2,0 = 5.2 × 10⁻⁵(T/298)^−5.28exp(−11 610/T) cm³ s⁻¹ and k_2,∞ = 1.2 × 10¹³(T/298)^0.06exp(−9800/T) s⁻¹, with overall error of ∼6% over the present range of temperature and pressure. The extrapolated k₂(298 K, 760 Torr) = 5.5^+9.2_−5.5 × 10⁻³ s⁻¹. The master equation calculations for CH₂OO indicate decomposition yields of 63.7% for H₂ + CO₂, 36.0% for H₂O + CO and 0.3% for OH + HCO with no significant dependence on temperature between 400 and 1200 K or pressure between 1 and 3000 Torr.