Two commercial TiO2 samples, a 100% anataseand a 100% rutile, were used for the fast field cycling NMRexperiments. The results showed a different behavior betweenthe different samples. In particular, water molecules wereunbonded to the solid surface for the rutile sample, whereasthey appeared to chemically interact with the surface throughH-bond formation with the anatase sample. The abovefindings accord with the generally lower activity of rutilewith respect to anatase reported in literature for photocatalyticoxidation reactions in water. The difficulty of water to interactwith rutile surface, indeed, could hinder the formation of OHradicals, which are the most important oxidant species.