For the high precision isotope analysis of atmospheric trace gases a computer controlled concentration interface has been developed. From small air samples it collects either N2O or CO2derived from CH4at their respective concentrations (0.3 ppm for N2O, 1.7 ppm for CH4) into a small diameter cold trap (−196°C) and interfaces via GC and open split to an isotope ratio mass spectrometer (Finnigan MAT 252) for on-line isotope evaluation. External reproducibilities for repeated measurements of 100 ml air samples from the same source of < 0.2° (δ-notation) have been achieved for13C/12C from CH4and for15N/14N and18O/16O from N2O. The precision is adequate to monitor the isotopic changes in these gases during a day's course.