<p>Clays often poses challenges to geotechnical engineers with its varying mineralogical conditions. Montmorillonite and kaolinite minerals are the most common minerals present in clayey soil, both of which have contrasting behavioural patterns. Clays are often analysed in tandem with pore fluids which play an important part in altering the behaviour of clays. Bentonite clays are often used as clay liners which expose it to leachates containing cations which can alter its behaviour. Naturally occurring clays with montmorillonite content are exposed to sea water which can alter its swelling behaviour. The swelling behaviour is also affected by the presence of kaolinite content in bentonite clays. In this study, the effect of salinity of sea water on swelling behaviour of montmorillonite clays with varying kaolinite content is investigated. The free swell behaviour of five clay samples were investigated with sea water of varying salinity. Optical microscopy images were used to confirm the particle size variation of the clays in the presence of the pore fluids. The results showed that the free swell index decreased with increase in salinity of sea water for montmorillonite rich samples while kaolinite rich samples showing little to no effect. Optical microscopy images observed an increased flocculation behaviour in montmorillonite rich samples while kaolinite rich samples showed no change in particle size with varying salinity. The drop of swell behaviour with increasing salinity in montmorillonite rich samples was attributed to collapse of diffuse double layer.</p>