The solvation of Cu2+ ions in sulfonated poly(styrene-co-divinylbenzene) ion-exchange resins has been studied by electron spin resonance (ESR) spectroscopy. The ESR spectrum of Cu2+ ions in hydrated resins is characteristic of the freely tumbling hexahydrated cation. On dehydration of the gel, the spectrum gradually broadens suggesting that dehydration occurs in a relatively disordered way. Reversible loss of ESR signal intensity is attributed to dipolar interactions between neighbouring Cu2+ ions. In resins fully solvated with methanol, the ESR spectrum is characteristic of the unconstrained, fully solvated Cu2+ ion and, again, on drying, loss of solvent occurs without the formation of any well-defined Cu2+ centres. Similar behaviour is seen for resins solvated with i-propanol and n-butanol.