The paper presents the whole spectrum of topics related to the determination of gases content in Karat apparatus. The construction of the apparatus is presented, likewise technical instruction of measurement is described. The paper presents also theoretical grounds of calculations of gases content in liquids by the use of values obtained on this apparatus and there is also described precision of the obtained values. Particular attention was paid to the method of calculation of the carbon dioxide content in water, which was introduced by Fresenius and Grünhut. All existed data are based on this method. Author has critically analysed this method and proposed the new theoretical grounds of calculations. These calculations takes into account new, additional paremeters, like: atmospheric pressure, difference of carbon dioxide concentration in pushed-out water and solobility of atmospheric gases in the water. Author made an analysis of the accuracy of this new method of determination the carbon dioxide concntent in water, taking into account the model of water degassing, uncertainty of the measurement and the way of samples collection. Author has made also the estimation of the influence of particular measured values on the measurement error. The new method of calculation gives higher values of measurements, then the method proposed by Fresenius and Grünhut. Author’s method doesn’t require to apply any empirical improvements leading to increase of the values obtained from theoretical calculations of the gas concentration in the water.