Cupric ions have been introduced into siliceous MCM-41 by liquid and solid state ion exchange. Electron spin resonance and electron spin echo modulation techniques were used to study the coordination of Cu(II) in dehydrated Cu-MCM-41 and its interaction with polar adsorbates such as D2O and NH3. This study reveals major differences in the location of cupric ions introduced by liquid or solid state ion exchange, Cupric ions exchanged by liquid state reaction are in a distorted tetrahedral geometry coordinated presumably to four framework oxygens in dehydrated Cu-MCM-41. After dehydration some copper ion pairs are observed; Polar adsorbates like water and ammonia remove the Cu(II) from the ion exchange site to form [Cu(H2O)(6)](2+) or [CU(NH3)(4)](2+) complexes, indicating only a weak interaction of the cupric ion in the ion exchange site with the MCM-41 framework. Cu(II) ions introduced by solid state ion exchange do not show any interaction with the MCM-41 framework. After dehydration the cupric ions are replaced by copper clusters which indicates that the cupric ions introduced by solid state ion exchange occupy different sites than by liquid phase ion exchange. However, similar interactions with adsorbates are observed in both solid state and liquid state ion-exchanged Cu-MCM-41.