Abstract: The invention concerns a process for washing fibrous materials, skins, textile materials or the like using an aqueous, surfactant containing washing liquid obtained by adding washing agents to water. The minimal dispensed amounts as determined in subsequent washing tests are always obtained automatically, without the cleaning power and the cleaning efficiency being reduced to unacceptable levels. To this end, the surface tension of the washing liquid is measured during the washing process using a tensiometer. The actual washing agent concentration is derived from these measured values and an initial concentration is derived from the quantity of washing agent added. The detergency is calculated from this initial concentration. The addition of a washing agent is discontinued when no further increase occurs in the detergency. The invention also concerns a device for carrying out the process. The device has capillaries connected to constant gas flow sources and immersed to the same depth in the washing liquid.