The goal of the present study was to examine the influence of CO2 concentration in the air ofindoor spaces on human well-being and intensity of mental work. Ten experimental subjectswere used in four experimental conditions with different CO2 concentrations (600, 1500,3000, 4000 ppm). Microclimatic parameters (CO2 concentration, temperature and relativehumidity of the air, surface temperature of walls) were measured. Well-being of subjects hasbeen evaluated with the aid of subjective scales, physiological variables were recorded andmental performance of subjects was measured by a standard test. It was checked that indoorclimate parameters- temperature, relative humidity - remained the same under exposure ofdifferent CO2 concentrations. Thus, the obtained results are related to the effects of CO2.Results revealed that human well-being as well as capacity to concentrate attention aredeclining when CO2 concentration in the air is increasing up to 3000 ppm.