Air sampling for analysis of CO and its isotopic composition (13C, 18O, and 14C) has been performed at the alpine station Sonnblick (47 ¿N, 13 ¿E, 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 7Be, O3, 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% for Δ13C, 0 and 8% for Δ18O, and 8 and 20 molecules cm-3 STP for 14CO. CO and stable isotope data are compared with results from a three-dimensional model (TM2). Generally good agreement supports the CO, Δ13CO, and ΔC18O 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. ¿ 2001 American Geophysical Union