Flux decline during nanofiltration of aqueous solutions containing dissolved organic compounds is mainly caused by adsorption of these compounds in the membrane pores and on the membrane surface. In this paper, flux decline is modeled by incorporating the loss of permeability due to adsorption in the Spiegler-Kedem equation. This results in a logarithmic relation between normalized flux decline and time until the adsorption equilibrium is reached and the flux reaches a constant value. In this way, the expected flux decline due to adsorption of organic compounds can be estimated. Two different parameters were used in the model: the time delay t(0), which corresponds to the time at which the adsorption process in the membrane pores and on the membrane surface sets in, and the corresponding reduction of free (pore) volume (b). Both parameters depend on the hydrophobicity of the compounds and on the feed concentration.