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 filter means, oxidation means and polishing means and further comprising means for measuring the resistivity of the water at the outlet of said filter means and at the outlet of said oxidation means, including the following steps: a) measuring the resistivity ρ UPW of the liquid at the outlet of the filter means; b) establishing a reference mode by exposing a portion of the liquid to said oxidation means for a predetermined number of significantly different time periods and measuring the corresponding resistivity values of the liquid after its exposure to said oxidation means; c) determining in this reference mode the resistivity at infinity ρ ∞REF of said liquid by establishing by regression a formula expressing its resistivity ρ as a function of time t, said formula including at least one exponential function of the type ρ = ρ ∞REF + (ρ UPW - ρ ∞REF) e -t/T; d) establishing an analysis mode by passing said liquid to be analyzed through one of said resistivity measuring means to determine its resistivity ρ at the outlet from the oxidation means; e) determining the purity of the water from the measured resistivity ρ, the resistivity ρ UPW of the water at the outlet from the filter means and the resistivity at infinity ρ ∞REF estimated in the reference mode; characterized in that said step e) of determining the purity includes the following sub-steps: - determining the time constant T of the exponential function established in the step c); - determining the resistivity at infinity ρ ∞ of said liquid in this analysis mode by successive iterations until this parameter converges, expressing the measured resistivity ρ as a function of this resistivity at infinity ρ ∞ by a formula including at least one exponential function of the type ρ = ρ ∞ + (ρ UPW - ρ ∞) e -t/T, t being an iteration variable, and by varying the iteration variable t toward infinity; - determining the quantity of organic compounds contained in said purified liquid from this resistivity at infinity ρ ∞. a) measuring the resistivity ρ UPW of the liquid at the outlet of the filter means; b) establishing a reference mode by exposing a portion of the liquid to said oxidation means for a predetermined number of significantly different time periods and measuring the corresponding resistivity values of the liquid after its exposure to said oxidation means; c) determining in this reference mode the resistivity at infinity ρ ∞REF of said liquid by establishing by regression a formula expressing its resistivity ρ as a function of time t, said formula including at least one exponential function of the type ρ = ρ ∞REF + (ρ UPW - ρ ∞REF) e -t/T; d) establishing an analysis mode by passing said liquid to be analyzed through one of said resistivity measuring means to determine its resistivity ρ at the outlet from the oxidation means; e) determining the purity of the water from the measured resistivity ρ, the resistivity ρ UPW of the water at the outlet from the filter means and the resistivity at infinity ρ ∞REF estimated in the reference mode; - determining the time constant T of the exponential function established in the step c); - determining the resistivity at infinity ρ ∞ of said liquid in this analysis mode by successive iterations until this parameter converges, expressing the measured resistivity ρ as a function of this resistivity at infinity ρ ∞ by a formula including at least one exponential function of the type ρ = ρ ∞ + (ρ UPW - ρ ∞) e -t/T, t being an iteration variable, and by varying the iteration variable t toward infinity; - determining the quantity of organic compounds contained in said purified liquid from this resistivity at infinity ρ ∞.