The mixing ratio of formaldehyde (HCHO) has been determined for air samples collected at a moderately polluted continental site (J¿lich, Federal Republic of Germany) and for samples collected at coastal sites in Ireland and New Zealand. In addition the HCHO mixing ratio has been determined for air samples collected in the North and South Atlantic during a cruise of the F/S Meteor from Hamburg at 55¿N to Montevideo at 35¿S in October and November 1980. The HCHO mixing ratio in clean tropical marine-air is of the order of 0.2 parts per billion by volume, about 50% lower than predicted by current photochemical models that include CH4 oxidation as the only source for HCHO. In the mid-Atlantic, diurnal variations of the HCHO mixing ratio showing weak maxima during the early afternoon were occasionally observed. These variations may be attributed to the diurnal behavior of the photochemical and physical processes which determine the mixing ratio of HCHO during stable weather conditions. The results are discussed with regard to the currently accepted theory of the photochemistry of HCHO in the clean troposphere. It appears that the yield of HCHO formation through hydrocarbon photooxidation in the background marine troposphere is less than assumed from model considerations, most probably because of the lack of sufficiently high NO concentrations.