A cotton disc of known nature and characteristics, held in a grip, is immersed in a solution of surface active agent of known concentration. After displacement of air an penetration of the solution into the cloth, the cotton disc starts to sink. The wetting time is determined by measurement the interval between the moment of immersion of the cotton disc and the moment when it begins to sink. The wetting time is determined of two standard solutions and for each of five different concentrations and then of the surface active agent solution under test for five different concentrations. The three curves "wetting time / concentration" are plotted and compared.