This paper reports the development of a new experimental method suitable for the determination of the spreading velocity of surface active substances on a thin film of liquid substrate over a solid surface. Previous research showed that when oil and surfactants spread on the surface of a deep body of water, the spreading velocity is usually quite high, being in the range of several tens of centimeters per second. However, the spreading velocities of these substances on a thin water film are much slower. This is because that the spreading of an oil or a surfactant on water surface is not just a two dimension phenomenon, but a phenomenon involving the movement of water in a small depth below the surface. An additional process impeding oil or surfactant spreading in this situation is the interaction of the spreading molecules with the liquid/soild interface underneath water surface. The experimental method developed in this work uses a Wilhelmy balance to monitor the average spreading velocity of surface active substances on a thin water film over a porous solid substrate. The spreading of cellulose hydrophobization agents over wet paper web during the papermaking process is used as an example to demonstrate the capability of the method.