A high performance liquid chromatographic technique has been used to determine the CH2O mixing ratio of tropospheric air. The technique has demonstrated adequate sensitivity and stability for measurements of the low CH2O mixing ratios expected at elevated altitudes. CH2O mixing ratios from a series of air samples collected at the surface around J¿lich, West Germany averaged 1.3¿0.7 ppbv over the period August 1979 to February 1980. Air samples were also collected during several tropospheric aircraft flights up to about 5 km and CH2O mixing ratios ranged between 0.3 and 0.5 ppbv below and less than 0.1 ppbv above the inversion.