The invention relates to a method of monitoring a concentration of a halogenated disinfectant in swimming pools or the like and a device for implementing the method. The method comprises: a liquid-injection step of injecting the liquid to be analyzed into the analysis chamber; a reagent-injection step of injecting a reagent into the analysis chamber; a measurement step of measuring the liquid/reagent mixture by colorimetry; a processing step of processing the measurement in relation to data; and a verification step of verifying the colorimetric measurement when the measurement corresponds to the reagent being bleached or to an absence of reaction color.