The crystal lattice of an arachidic acid/Cd2+ (CdA) monolayer on the surface of aqueous CdCl2 subphase in a Langmuir trough was investigated in situ by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The spectra reveal a hexagonal packing of the hydrocarbon chains on the subphase of pH=6.2 and 293-283 K. Cooling from 283 to 280 K induces the hexagonal unit cell to transform to an orthorhombic one, as evidenced by the splitting of the CH2 scissoring mode. This transition is observed to be reversible with temperature variation within 293-274 K. On the subphase of pH=8.4, the crystal lattice of CdA is orthorhombic even at 293 K. The temperature-dependent crystal lattice transition of CdA is compared with the monotonic hexagonal subcell of a cadmium stearate monolayer within 293-274 K and the orthorhombic subcell of a cadmium behenate monolayer within 293-274 K. (C) 2001 American Institute of Physics.