FTIR spectroscopy has been applied to NH + 4-exchanged dioctahedral clay minerals to determine the molecular environment of NH + 4 and to quantify N concentration. FTIR under vapour-pressure control, coupled with heating and freezing treatments has shown that NH + 4 ion symmetry varies with the nature of clay minerals. NH + 4 has a perfect tetrahedral symmetry in hydrated or dehydrated smectites and belongs to the T d symmetry group. The NH + 4-bending vibration is centred at 1450 and 1425 cm-1. The Si 4+-Al 3+ substitution in dioctahedral clay minerals induces the loss of symmetry elements of the NH + 4 tetrahedron which acquires a C 2v symmetry. As a consequence, the T d-C 2v transition can be used to characterize the smectite-illite transition. Quantification of NH + 4 content per half unit cell is provided by n NH 4 = k[NH 4 ]/[OH] where [NH 4 ]/[OH] is the band area ratio of the NH + 4-bending vibration to the OH-stretching vibration. k = 1.1 for hydrated smectite, 0.9 for dehydrated smectite and 0.8 for illite or tobelite. The bending vibration of NH + 4 is chosen for the calculation because it is not affected by superimposed contributions.