The antiferro-/paraelec. (.hivin.42m/.hivin.43m at T1 = 264 K) and ferro-/antiferroelec. (mm2/.hivin.42m at T2 ~ 160 K) phase transitions of Cl-Cl boracite were studied by dielec. and thermal anal. The permittivity of both ferroic phases shows small and weakly temp. dependent values (er' = 8.5-13) and discontinuity at the onset of the phase transitions at T1 and T2, which are characterized by improper nature. Above T1 er' decreases with increasing temp., referring to the Curie-Weiss law. Microcalorimetry shows the relevant enthalpy changes: DH1 = 1.98 kJ.mole-1 at T1; DH... = 0.03 kJ.mole-1 at T2. The effect of the hydrostatic pressure on the phase transition temp. is discussed.