FTIR spectroscopy has been applied to NH₄⁺-exchanged dioctahedral clay minerals to determine the molecular environment of NH₄⁺ and to quantify N concentration. FTIR under vapourpressure control, coupled with heating and freezing treatments has shown that NH₄⁺ ion symmetry varies with the nature of clay minerals. NH₄⁺ has a perfect tetrahedral symmetry in hydrated or dehydrated smectites and belongs to the T_d symmetry group. The NH₄⁺-bending vibration is centred at 1450 and 1425 cm^–1.The Si⁴⁺-Al³⁺ substitution in dioctahedral clay minerals induces the loss of symmetry elements of the NH₄⁺ tetrahedron which acquires a C_2v symmetry. As a consequence, the Td –C2v transition can be used to characterize the smectite–illite transition. Quantification of NH₄⁺ content per half unit cell is provided by n_NH4 = k[NH₄]/[OH] where [NH4]/[OH] is the band area ratio of the NH₄⁺-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 NH₄⁺ is chosen for the calculation because it is not affected by superimposed contributions.