Rate coefficients for the reaction between NO₃ and acetaldehyde have been determined by the absolute rate fast-flow-discharge technique and by the relative rate method. The flow-tube experiments were carried out under pseudo-first-order conditions in NO₃ over the temperature range 263–363 K using He as a carrier gas. The data suggests an activation energy of E_a/R = 1950 ± 290 K and k₂₉₆ = (9.1 ± 0.8) × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹ (3 σ errors) as an upper limit. Mixing ca. 20% of oxygen in the He carrier gas resulted in E_a/R = 2020 ± 260 K and k₂₉₆ = (2.5 ± 0.5) × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹. The relative rate experiments, performed in a static reactor employing long path FTIR detection, gave k₂₉₈ = (2.62 ± 0.29) × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹, and showed a moderate kinetic isotope effect, k_CH3CHO+NO3/k_CH3CDO+NO2 = 2.37 ± 0.08 at 298 K. The differences in the reaction rate coefficients obtained by the two methods are analysed and discussed in terms of secondary reactions involving NO₃ in the flow-tube. Model studies indicate that acetyl and peroxyacetyl radicals react with NO₃ with rate coefficients of 2.5 × 10⁻¹¹ and 1.5 × 10⁻¹³ molecule cm⁻³ s⁻¹ at 296 K and 5 mbar, respectively. The reaction was also studied by quantum mechanical methods and the transition states for the abstraction of aldehydic and methylic hydrogen atoms were located. Their relative energies, calculated on the MP2/cc-pVDZ//CCSDT/cc-pVDZ level, conform to the reaction proceeding entirely through H_ald-abstraction at room temperature.