Very low H-O-H bending frequencies (appreciably lower than that in gaseous water) have been observed in the infrared spectra of several compounds of the MKPO 4 .H 2 O type. Thus, in the case of NiKPO 4 .H 2 O the only intense band attributable to the H-O-H bending appears at 1478cm - 1 , whereas in the case of the magnesium compound the corresponding frequency is 1470cm - 1 . A tentative explanation (supported by quantum chemical calculations) for this unusual phenomenon has been offered.