While isotope effects in formaldehyde photolysis are the key link between the δD of methane emissions with the δD of atmospheric in situ hydrogen production, the mechanism and the extent of their pressure dependencies is not adequately described. The pressure dependence of the photolysis rates of the mono- and di-deuterated formaldehyde isotopologues HDCO and D<sub>2</sub>CO relative to the parent isotopologue H<sub>2</sub>CO was investigated using RRKM theory and experiment. D<sub>2</sub>CO and H<sub>2</sub>CO were photolysed in a static reaction chamber at bath gas pressures of 50, 200, 400, 600 and 1000 mbar; these experiments compliment and extend our earlier work with HDCO vs. H<sub>2</sub>CO. The UV lamps used for photolysis emit light at wavelengths that mainly dissociate formaldehyde into molecular products, CO and H<sub>2</sub> or D<sub>2</sub>. A model was constructed using RRKM theory to calculate the lifetime of excited formaldehyde on the <i>S</i><sub>0</sub> surface to describe the observed pressure dependent photolytic fractionation of deuterium. The effect of deuteration on the RRKM lifetime of the <i>S</i><sub>0</sub> state is not the main cause of the experimentally observed isotope effect. We propose that there is an additional previously unrecognised isotopic fractionation in the rate of transfer of population from the initially excited <i>S</i><sub>1</sub> state onto the <i>S</i><sub>0</sub> surface.