Disclosed is a method 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 a filter, an oxidizing device and a polishing device, and further including a resistivity measuring device for measuring the resitivity of water to determine the purity thereof. Only one resistivity measuring cell is used, and the outlet points of the filter and the oxidizing device are connected to the resistivity measuring cell by pipes provided with an analysis valve and/or check valves selectively enabling circulation of the liquid within them.