The relative photolysis rates of HCHO, (HCHO)-C-13, (HCHO)-O-18, and DCDO were studied in pseudo-natural tropospheric conditions in July 2003 at the European Photoreactor Facility (EUPHORE) in Valencia, Spain. The photolytic decay of HCHO, (HCHO)-C-13, and (HCHO)-O-18 is measured relative to DCDO by long path FT-IR detection during the course of about 3 h of sunlight. The relative photolysis rates obtained are as follows: J(H)(CHO)(13)/J(HCHO) = 0.894 +/- 0.006, J(HCH)(O)(18)/J(HCHO) = 0.911 +/- 0.011, and J(DCDO)/J(HCHO) = 0.597 +/- 0.001. The errors represent la and do not include possible systematic errors. The atmospheric implications of the large isotope effects in the photolysis of formaldehyde are discussed.