This is a device for analyzing the quantity of organic compounds existing in a liquid, such as ultrapure water, at the outlet from a purification device including in series filter means (1), oxidation means (2) and polishing means (3), further including means for measuring the resistivity of water to determine the purity thereof, characterized in that it includes only one resistivity measuring cell (4) and in that the outlet points of said filter means (1) and said oxidation means (2) are connected to said resistivity measuring cell (4) by pipes provided with an analysis valve (6) and/or check valves (5) selectively enabling circulation of said liquid within them.