This chapter investigates the solvent extraction behavior of copper (Cu), cadmium (Cd), and zinc (Zn) from aqueous solutions using phosphonium-based ionic liquids as extractants. The extraction efficiency, selectivity, and mechanism are examined under varying conditions including pH, metal ion concentration, and the nature of the phosphonium ion. The study highlights the significant role of phosphonium ionic liquids in enhancing the separation and recovery of these metal ions due to their unique chemical properties, such as high thermal stability, low volatility, and strong metal-binding capabilities. Experimental results demonstrate that phosphonium ions exhibit selective extraction efficiencies for Cu, Cd, and Zn, with Cu showing the highest affinity under optimized conditions. The chapter also discusses the thermodynamics and kinetics of the extraction process, providing valuable insight into the potential industrial applications of phosphonium-based solvent extraction systems in hydrometallurgy and wastewater treatment.