New evidence for the framework substitution of titanium into TiMCM-41 mesoporous molecular sieve is found from electron spin resonance (ESR) studies. TiMCM-41 samples with varying Si/Ti ratios have been synthesized as have titanosilicate TS-1, siliceous MCM-41, AlMCM-41, and Ti-AlMCM-41 in the last of which titanium ions are incorporated by ion exchange. These materials were examined by ESR spectroscopy after a thermal activation treatment followed by gamma-irradiation at 77 K. gamma-Irradiation at 77 K of TiMCM-41 having a high Si/Ti ratio produces a strong orthorhombic ESR signal due to V centers and an axial signal with g(parallel to) = 1.971 and g(perpendicular to) = 1.901 best explained as arising from trivalent titanium situated at a framework tetrahedral site. An upper limit for the substitution of titanium into a framework tetrahedral site is observed. As the titanium concentration in the synthesis gel increases, the resulting material contains both tetrahedral and octahedral titanium. gamma-Irradiation of ion-exchanged Ti-AlMCM-41 gives an axial ESR signal characterized by reverse g values g(parallel to) = 1.898 and g(perpendicular to) = 1.967 typical of Ti(III) in distorted octahedral symmetry. Both TiMCM-41 and Ti-AlMCM-41 differ in their behavior toward interaction with various adsorbate molecules. The Ti(III) species observed in TiMCM-41 after adsorption of D2O, CO, NH3, and CD3OH changes its ESR characteristics from that of the original Ti(III) species observed in the absence of these adsorbates. The ESR characteristics of the Ti(III) species in Ti-AlMCM-41 after adsorption of the above adsorbates are different from those of TiMCM-41, indicating that the Ti in these two materials are in different coordination environments. This independently supports the different g tensors for Ti(LII) in TiMCM-41 versus TiAlMCM-41.