Continuous measurements of alkyl nitrates were made during the Tropospheric Ozone Research subproject of the European Experiment on Transport and Transformation of Environmentally Relevant Trace Constituents in the Troposphere Over Europe at the Schauinsland station in the Black Forest between June 1990 and May 1991, using an automated gas chromatograph with a NOy detector. More than 2000 samples were analyzed with a sampling frequency of six to nine samples per day. The mixing ratio of the sum of alkyl nitrates (C1--C8) ranged between 30 and 630 parts per trillion (ppt) and averaged 120 ppt over the whole measurement period. The average contribution of alkyl nitrates to total odd nitrogen was about 3%, with little seasonal variation. The most abundant individual nitrates were 2-propyl, 2-butyl, and methyl nitrate. When lumped by carbon number, the sum of C5 nitrates represented the largest fraction after methyl nitrate. The mixing ratios of nitrates ≤C7 were very small. In polluted air masses, the mixing ratios of alkyl nitrates exhibited summer maxima and pronounced daily variations, with maxima in the early evening. The abundance increased from C3 to C5. In clean, photochemically aged air masses, alkyl nitrate mixing ratios showed a summer minimum and no pronounced diurnal variations. A decrease of abundance from C3 toward higher carbon chain lengths was observed, as is expected for air that is characterized by photochemical destruction. The measured concentration ratios between different alkyl nitrate isomers are utilized to resolve discrepancies in the existing data on branching ratios for the formation of alkyl nitrates from the reaction of peroxy radicals with NO and to classify air masses with respect to photochemical age. The observed behavior and mixing ratios of methyl and ethyl nitrate cannot be explained by formation solely from the reaction of peroxy radicals and NO. ¿ 1998 American Geophysical Union