Air sampling for analysis of CO and its isotopic composition (C-13, O-18, and C-14) has been performed at the alpine station Sonnblick(47 degreesN, 13 degreesE, 3106 m above sea level) since September 1996. A high degree of variability is observed, which is due to the wide variation in the origin of air masses sampled. On the basis of the CO and isotope results, a classification of the different samples is performed. Other data such as Be-7, O-3, relative humidity, and back trajectories are used to give additional information about the air mass origin. Background values, representative of the NH midlatitudes free troposphere, are observed about 50% of the time and are used to define seasonal cycles. CO and its isotopes show a minimum in summer and a maximum in winter with extreme values of 90 and 160 ppb for CO, -30 and -25 parts per thousand for delta C-13, 0 and 8 parts per thousand for delta O-18, and 8 and 20 molecules cm(-3) STP for (CO)-C-14. CO and stable isotope data are compared with results from a three-dimensional model (TM2). Generally good agreement supports the CO, delta (CO)-C-13, and deltaC(18)O source/sink distributions inferred by the model. According to model calculations, fossil fuel combustion contributes 35% in summer and 50% in winter of total CO for such a midlatitude location. Other categories of sampled air are "subtropical," "polluted," and "stratospheric" and are observed 24%, 18%, and 4% of the time. Corresponding signatures of CO and its isotopic variations are presented, and some specific events are discussed.