Liquid–liquid extraction with PEG-based ATPS is a promising strategy for the recovery and separation of metal ions from aqueous media replacing conventional organic-aqueous systems from an environmental point of view. Although ATPS extraction methods using an organic ligand that coordinates metal ions have been investigated since this technique is commonly used in conventional liquid–liquid extraction of metal ions, the use of this method for actual analytical or industrial separations is very limited because most of the organic extracting agents are hydrophobic and do not dissolve in water. On the other hand, extraction of metal ions as negatively charged complexes of halide and pseudohalide ions has been proven to be effective in PEG/salt ATPS. In the present chapter, the mechanism of the extraction of metal ions in PEG/Na2SO4 ATPS is discussed taking iodide and thiocyanate ions as the extracting agents based on the complexation of the metal ions not only with iodide or thiocyanate ion but also with the phase-forming sulfate ion. Simple and green extraction-spectrophotometric methods using PEG/Na2SO4 ATPS developed for on-site analyses of heavy metal ions in environmental samples (river water, soil, and sediment) will also be presented.