An improved analytical procedure has been developed for the detection of formaldehyde, acetaldehyde, acetone and other volatile carbonyls in clean air. For sampling, 2,4-dinitrophenyl-hydrazine (DNPH) coated silica gel cartridges were used. DNPH reacts with carbonyls and forms carbonyl hydrazones which are extracted with acetonitrile and subsequently separated by reversed phase HPLC. Sampling flow rates up to 3.5 l/min were tested. The quantification limit of the complete sampling and analytical procedure is 60 ng carbonyl which corresponds to a mixing ratio of 1 ppbv HCHO in a 45 l air sample taken during a sampling time of 13 min. Carbonyl mixing ratios down to 0.1 ppbv can be determined. The collection efficiency and the elution recovery range between 96 and 100%; the precision is ±5% for HCHO and ±4% for CH₃CHO at mixing ratios of 1 ppbv. This technique can also be applied for the determination of aldehydes and ketones in the aqueous phase, e.g. cloud and fog water. In this case, carbonyls were converted to hydrazones simply by mixing the aqueous sample with an acidified DNPH solution. After 40 min reaction time, the hydrazones were analysed by HPLC. The detection limit was 0.2 μmol HCHO/l. Possible interference caused by ozone and NO₂ was eliminated by using KI filters connected in series with the DNPH-coated cartridges. The analytical procedure was tested at a mountain measuring station and proved to be a suitable method for monitoring carbonyl compounds in clean air.