This paper aims to develop a completely theoretical approach for assessing scaling (crystallization/precipitation fouling) propensity through the previously proposed and tested theoretical index of 2nd author (SPI) within a Reverse Osmosis (RO) module. This approach will incorporate the local varying foulants' properties along membrane filtration channel into fundamental transport and conservation equations to achieve a unified and scientific assessment of the scaling potential. Thus, a high accuracy simulation of the local fouling propensity along a full-scale RO module will be obtained. This paper has overcome the challenge of determining and incorporating the locally varying concentration of foulants (and factors affecting it) and effect of concentration polarization to predict the local fouling propensity along the membrane channel. Also it has incorporated the only available theoretical index without any need for empirical parameters and/or constant Therefore, a fundamental more realistic description of fouling propensity and onset of fouling next to membrane along feed channel is developed and explained. Moreover, the effects of initial applied pressure, initial cross flow velocity, initial feed water salinity, clean membrane resistance and feed water temperature on fouling propensity and onset of fouling along the channel are investigated and discussed. Our suggested model is a very powerful tool that could help in the specific design of RO process as well as in simulations of the operating variables for optimization of RO system. (C) 2011 Published by Elsevier B.V.