Abstract: The effectiveness of cement-fly ash-treated contaminated soils in two remediation case studies in the UK is studied by comparing their time-related performances (e.g., strength, pH and leachate concentration of heavy metals). This is because the long-term performances of in-situ stabilisation/solidification technology and the effects of this technology on different contaminated sites are uncertain. The sites involved in this research are the West Drayton in Middlesex and the Castleford, Yorkshire, each with a different range of organic and inorganic contaminants (Cu, Ni, etc.). Field trial samples are subjected to unconfined compressive strength (UCS) and two batch leaching tests (TCLP and BSEN 12457) at different time points. The results detail the strength and pH relationship of similar mixes under different site conditions and different time scales, so as to validate the effectiveness of this remediation technique. The immobilization degrees of these two metals in different mixes under different time points (0.08~17 years) are calculated, the values of which are >99.4%. The 17 years’ effectiveness of cement-fly ash-stabilised/solidified contaminated soils in these two case studies is confirmed by comparing their strengths and leachabilities at different curing time points.