The present study aims to investigate the viability of using Spiegler-Kedem model to predict the retention of atrazine and dimethoate with nanofiltration using NF90 in stirred cell condition. Spiegler-Kedem model is the thermodynamics of irreversible processes in which no particular mechanism of transport and structure of membrane is specified. The Spiegler-Kedem transport equations were used to derive the reflection coefficient and solute permeability of the system. The model was successfully applied on the modeling of the organic molecules tested. It was found that Spiegler-Kedem model provided a good estimation of experimental value. The coefficient of determination (R-2) obtained for the fitted data was 0.9871 and 0.9692 for atrazine and dimethoate, respectively. (C) 2008 Published by Elsevier B.V.