A comparison of alginate fouling in forward osmosis (FO) with that in reverse osmosis (RO) was made. A key experimental ﬁnding, corroborated by membrane autopsies, was that FO is essentially more prone to fouling than RO, which is opposite to a common claim in the literature where deductions on fouling are often based solely on the water ﬂux proﬁles. Our theoretical analysis shows that, due to a decrease in the intensity of internal concentration polarization (ICP), and thus an increase in the eﬀective osmotic driving force during FO fouling tests, the similarity of experimental water ﬂux proﬁles for FO and RO is in accordance with there being greater fouling in FO than RO. The speciﬁc foulant resistance for FO was also found to be greater than that for RO. Possible explanations are discussed and these include the inﬂuence of reverse solute diﬀusion from draw solution. Whilst this explanation regarding speciﬁc foulant resistance is dependent on the draw solution properties, the ﬁnding of greater overall foulant accumulation in FO is considered to be a general ﬁnding. Additionally, the present study did not ﬁnd evidence that hydraulic pressure in RO plays a critical role in foulant layer compaction. Overall this study demonstrated that although FO has higher fouling propensity, it oﬀers superior water ﬂux stability against fouling. For certain practical applications this resilience may be important.