In the photolysis of atmospheric hydrocarbons, the photolysis of formaldehyde is a key step. The photolysis proceeds via two channels, often referred to as the radical channel (1) and the molecular channel (2). (1) HCHO + hv (λ < 340 nm) → H + HCO; (2) HCHO + hv (λ < 360 nm) → H₂ + CO There is a great interest in the different steps of the hydrocarbon oxidation since the chain of reactions, CH₄ → CH₃ → CH₃O₂ → CH₃O → HCHO → CO+H₂, is a link between the carbon- and hydrogen cycles in the atmosphere. A recent work report the relative natural sunlight photolysis rates of HCHO and HCDO measured at the European Photoreactor Facility (EUPHORE) in Valencia, Spain. In a laboratory study at the University of Copenhagen HCHO and HCDO have been photolyzed in an indoor reaction chamber. The temperature and pressure dependence of the isotope effect has been studied.