Medium-pore silicoaluminophosphate type 41 (SAPO-41) molecular sieve has been synthesized by hydrothermal methods. The reducibility of Ni(II) ions ion-exchanged into SAPO-41 (NiH-SAPO-41) by various reduction methods was studied by electron spin resonance spectroscopy. Dehydration of NiH-SAPO-41 at temperatures above 573 K produces an isolated Ni(I) species with g(parallel to) = 2.498 and g(perpendicular to) = 2.115. When dehydrated and oxidized NiH-SAPO-41 is treated with hydrogen, more than one isolated Ni(I) species are observed depending on the reduction temperature. When NiH-SAPO-41 is gamma-irradiated at 77 K, Ni(I) is also formed. Adsorption of D2O into dehydrated NiH-SAPO-41 forms Ni(I)-(O-2)(n), indicating water decomposition by Ni(I). Adsorption of ammonia on reduced NiH-SAPO-41 leads to Ni(I)-(ND3)(n), with similar ESR characteristics as that of the Ni(I)-(O-2)(n) complex produced by water adsorption. The similar ESR spectra of these two species suggest that both species are localized at the same site in the molecular sieve structure and have similar symmetry with two or three molecular ligands. Methanol adsorption on NiH-SAPO-41 forms Ni(I)-(CH3OD)(I) with axially symmetric g values. Adsorption of ethylene on NiH-SAPO-41 reduces most Ni(I) ions formed during hydrogen reduction to Ni(0). The location of Ni(I) in NiH-SAPO-41 was determined by P-31 electron spin echo modulation and is located in the main 10-ring channel near a six-ring window.