The structures of CHF3 and CH3Cl monolayers adsorbed on the basal plane of boron nitride have been examined by X-ray diffraction. For CHF3, orientationally disordered, triangular, incommensurate high-temperature and orientationally ordered low-temperature incommensurate phases were observed. The latter phase has antiferroelectric ordering of an electric dipole moment and is uniaxially compressed. On the other hand, CH3Cl molecules form only one crystalline monolayer of an antiferroelectric structure, independent of temperature and coverage. These results are compared with the monolayers of these molecules on graphite.