This paper aims at establishing the influence of acidic and alkaline pore fluids on the swell behaviour of an expansive soil. A series of laboratory one dimensional free swell tests were performed to study the behaviour of soil in acidic and alkaline environment. Three different concentrations of sodium hydroxide and sulphuric acid solutions were used as pore fluids to understand the influence of variable concentrations on the swell behaviour of soil. Results showed that, the swelling of soil that interacted with sodium hydroxide solution initially increased at lower concentration and then decreased with increase in concentration. In contrast, the swelling initially decreased at lower concentration of sulphuric acid and then increased with increase in concentration of solution. The complexity in the swell behaviour of contaminated soil was assessed by thoroughly investigating the mineralogy and microstructure alterations by carrying out X-ray diffraction analysis, Scanning electron microscopy and Energy dispersive analysis of X-ray at the end of interaction. © 2017, Budapest University of Technology and Economics. All rights reserved.