High-resolution P-31 nuclear magnetic resonance (NMR) techniques were employed to study a KH2PO4-type ferroelectric system, TIH2PO4. A marked temperature dependence of the isotropic chemical shift below the ferroelastic phase transition temperature is indicative of an electronic instability. The NMR linewidth showed a discontinuity at the ferroelastic phase transition, and the anisotropy was measured to increase rapidly below the antiferroelectric phase transition. Thus, the changes in the microscopic environments associated with the phase transitions were sensitively reflected in a characteristic manner. (C) 2003 Elsevier B.V. All rights reserved.