The H+-ion treatment effect on TIH2PO4, a KH2PO4 (KDP)-type ferroelectrics, was studied by nuclear magnetic resonance (NMR) and AC dielectric constant measurements. A sample of TIH2PO4 was irradiated by 1-MeV H+ ion beams to a dose of 10(15) ionS/cm(2). The change in the NMR relaxation behaviors after the irradiation was attributed that in the hydrogen-bond geometry, presumably affecting the order-disorder proton dynamics. A prominent decrease in the dielectric constant was also observed after the irradiation. The macroscopic and microscopic changes due to the irradiation are discussed in the light of the proton dynamics. (c) 2005 Elsevier B.V. All rights reserved.