Swelling elastomers possess the unique ability to expand upon contact with fluids such as water and oil. They are used in key applications in oil and gas drilling, such as water-shutoff and zonal-isolation. Rubber swelling depends on its chemical, structural, and mechanical properties. The polymer-fluid interaction parameter (chi), also known as the Flory-Huggins parameter, plays a crucial role in governing the swelling process. Traditionally, determining chi involved chemically dissociating the polymer in the swelling medium, a complex and specialized method. This paper introduces a novel approach to estimate chi through swelling and mechanical testing. Tests were conducted on two different elastomers that were submerged in saline water solutions with concentrations of 0.6% and 12%, maintained at a temperature of 50 °C for a duration of 1 month. Measurements were taken prior to swelling and at intervals of 1, 3, 6, 9, 15, 22, and 30 days after swelling. Swelling and mechanical tests were performed to observe changes in mass, volume, density, elastic modulus, and bulk modulus. Mass and volume exhibited an increasing trend with swelling time, while density demonstrated some fluctuations. Young’s modulus experienced a rapid initial decrease of approximately 90% within the first 1–2 days of swelling, after which it remained relatively constant. Bulk modulus displayed fluctuating behavior but exhibited a significant decrease over the 30-day period. Determination of the shear modulus took into account the isotropic relationship and was subsequently utilized to ascertain chain density. Density, volume swelling ratio, molar volume, shear modulus, and average chain molecular weight were employed to calculate the chi value. Both rubber materials exhibited higher chi values in high salinity brine compared to low salinity brine. Also, chi values for both materials decreased as the swelling progressed, in line with previously published literature. Elastomer immersed in low salinity brine solution experienced a more substantial decrease in chi value. The findings of this paper hold significant importance for field engineers, scientists, and application developers working with swellable elastomers, as well as the broader fields of rubbers and polymers in general.