Ni(II) ions have been ion-exchanged into mesoporous MCM-41 and AlMCM-41 materials by liquid-(L) and solid-state (S) reaction. The first isolated Ni(I) species stabilized in these materials can be generated by thermal or hydrogen reduction and by gamma-radiation in (L)Ni-AlMCM-41, where Ni(II) has been introduced by liquid-phase ion exchange. Reduction by gamma-radiation is the most effective method, and in (L)Ni-AlMCM-41 the formation of two different Ni(I) species with g(parallel to) = 2.502, g(perpendicular to) = 2.091 and g(parallel to) = 2.384, g(perpendicular to) = 2.129 is observed. However, in (L)Ni-MCM-41 and (S)Ni-AlMCM-41 no stabilized Ni(I) species could be detected.