The mechanical behaviour of clays is of importance to geotechnical engineers since the advent of the soil mechanics. The analysis of clays is seldom done in dry or partially saturated state and pore fluids play a vital part of the soil matrix. The investigations on clays have been always limited to mechanical behaviour with distilled water as pore fluids. In practical case, the pore fluid is seldom distilled water. In marine environments, the pore fluid would be in effect a salt solution which has much lesser dielectric constant than that of distilled water. The swelling behaviour of clays have always been of predominant importance in analysis of clays and the related foundation design on swelling clays. Bentonite clays are often used for waste containment barriers and deep geological repositories which may make them come in contact with organic fluids which can affect there swelling behaviour which in turn affects the integrity of the clay barrier. The swelling behaviour of clays have always been of predominant importance in analysis of clays and the related foundation design on swelling clays. In this paper, the free swell behaviour of clays has been investigated by using organic fluids with varying dielectric constants. Bentonite and kaolinite clays have been used in the investigation and the free swell is calculated by using five pore fluids, namely: Formamide, Distilled water, 60% ethanol and 100% ethanol. Optical microscopy investigation was also used to assess the behaviour characteristics of the clay pore fluid combinations. The free swell potential of bentonite-rich clay samples was observed to decrease with change in dielectric constant of pore fluid while kaolinite-rich samples showed much little effect to variation of dielectric constant. Optical microscopy images showed particle size variation which correlated with the free swell variation of all clay samples.