The article describes monitoring and analysis of CO2 concentration in the Bijambare cave in Bosnia and Herzegovina. CO2 concentrations were interpreted with the cave climatic modl. During the cold period, ventilation decreases the level of CO2, during warmer period, the CO2 in the cave accumulates. We developed a simple first order reactor model to intepret the CO2 time series. Using this model, we evaluated different sources of the cave CO2.