A thermal method for the determination of ambient organic carbon (OC) and elemental carbon (EC) concentrations in carbonaceous samples was further developed. Possible artifacts were investigated and were shown to be low. Good agreement of EC data with the German VDI reference method was found and detection limits were 1.3 μg for EC and 1.8 μg for OC. The method was applied to samples obtained with an aethalometer from an ongoing campaign at the high-alpine research station Jungfraujoch from July 1995 to June 1997. Measurements of EC concentration were used to derive a new site-specific calibration factor (instrumental absorption efficiency α(API)) for the determination of the black carbon (BC) concentration. Despite a distinct seasonal cycle in BC, of around one order in magnitude with a maximum in summer and minimum in winter, α(API) exhibited no significant seasonality. The derived calibration factor for the Jungfraujoch, α(API) = 9.3 ± 0.4 m² g^-1, is lower than the manufacturer calibration by a factor ~2. The results confirm the observation that the aethalometer determined BC concentration, underestimates the true value at remote sites, when the manufacturer calibration is used.