A quantitative estimate of dipolar interaction energy causing the order–disorder ferroelectric phase transition in triglycine sulphate (TGS) is obtained and also the effect of small hydrostatic pressures on the dipolar energy is modelled. At high pressures of the order of 2.5 GPa a crystallographic phase transition to a non-ferroelectric phase takes place in these crystals, the spontaneous polarization goes to zero most likely because of the glycine ion GI becoming planar at high pressures. The inverse Ubbelohde effect observed in TGS is successfully explained using the corrected valence bond model. The increase in transition temperature (T_C) on deuteration is associated with the increase in value of the reorientable dipole moment of TGS. A quantitative estimate of increase is obtained using the Mecke dipole function