The structure of the model layered compound of [C(NH2)(3)](+ .) NO3-has been refined using high-resolution powder neutron diffraction data collected at 4 and 395 K. While in the low-temperature the hydrogen bond geometry is typical of the medium-strength heteronuclear hydrogen bonds, at high-temperature the H-atoms shift to the centres between the N and O atoms. The H-centring in NH--O hydrogen bonds has been rationalized in terms of thermal activation of the hydrogens in the diabatically modified potential-energy wells, as well as of specific features of diffraction data scattered of a strongly vibrating structure. (C) 2004 Elsevier B.V. All rights reserved.