In October 1987 we made preliminary studies of concentration of soil-CO2and its isotopic composition. The sample-size of soil air was about 100 cm3only, which is significantly reduced in comparison with other studies so far. The isotope analysis of such minute CO2samples were performed by a dynamic flow mass spectrometer. The air samples were taken from the depth of 10 … 75 cm at two points of a grass field, which were distanced a few meters apart. The measured δ13C values were very consistent for both points and they were ranging from –21.53 … –22.37‰ while the corresponding CO2concentration was ranging from 0.24 … 0.55%.