Chelex 100 resin is a styrene divinylbenzene copolymer containing paired iminodiacetate ions, which act as chelating groups in binding polyvalent metal ions. The carboxylic acid groups of Chelex 100 resin classify it as a weak cation exchange resin, but it differs from other exchangers in this class by featuring uniquely high selectivities for metal ions and much higher bond strengths. The resin can be used to ultrapurify buffers and ionic reagents: it will scavenge metal contaminants without altering the concentration of nonmetal ions.

Chelex 100 chelating ion exchange resin has a stronger selectivity for copper, iron, and other heavy metal ions over monovalent cations such as sodium and potassium. Its selectivity for divalent over monovalent ions is approximately 5,000 to 1, and it has a strong attraction for transition metals, even in highly concentrated salt solutions.