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. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.