Process for determination of a water content material, in particular organic carbon content and/or nitrogen, in which an aqueous sample is evaporated and combusted in at least one heating vessel equipped with a heating facility and the combustion product is fed in a transport gas flow to a detector for determination of the concentration of a gaseous compound of the water content material, characterised in that calibration is performed with a predetermined volume of a calibration gas which contains a predetermined concentration of the element corresponding to the water content material, in particular carbon and/or nitrogen.