This article presents a lab experience to study detergency from a physicochemical point of view intended for undergraduate students. By means of a simple experimental device, we analyze the influence of the surfactant concentration in both distilled water and tap water. Our method is based on the measurement of diffuse reflectances of polyester samples impregnated with linseed oil. The discussion of the experimental results helps to introduce physicochemical concepts such as surfactant, adsorption, surface tension, work of adhesion, Gibbs isotherm, and electrical double layer, emulsion, or micelle. The device and the methodology adapt easily to study the influence of other experimental parameters such as temperature, stirring intensity, washing time, type of surfactant, and so forth. To extend the range of applicability of this experience we have included, in the Supplemental Material, detailed information about the syntheses of the organic and inorganic components of the detergent, analytical techniques to measure water hardness, surface tension measurements, and the mathematical treatment of experimental errors. (Contains 3 figures.)