The compounds M[PO 2(OH) 2] 2·2H 2O ( M=Mg, Mn, Fe, Co, Ni, Zn, Cd) were prepared from super-saturated aqueous solutions at room temperature. Single-crystal X-ray structure investigations of members with M=Ni, Zn, Cd were performed at 295 and 120 K. The space-group symmetry is P2 1/ n, Z=2. The unit-cell parameters are at 295/120 K for M=Ni: a=7.240(2)/7.202(2), b=9.794(2)/9.799(2), c=5.313(1)/5.285(1) Å, β=94.81(1)/94.38(1)°, V=375.4/371.9 Å 3; M=Zn: a=7.263(2)/7.221(2), b=9.893(2)/9.899(3), c=5.328(1)/5.296(2) Å, β=94.79(1)/94.31(2)°, V=381.5/377.5 Å 3; M=Cd: a=7.356(2)/7.319(2), b=10.416(2)/10.423(3), c=5.407(1)/5.371(2) Å, β=93.85(1)/93.30(2)°, V=413.4/409.1 Å 3. Layers of corner-shared MO 6 octahedra and phosphate tetrahedra are linked by three of the four crystallographically different hydrogen bonds. The fourth hydrogen bond (located within the layer) is worth mentioning because of the short O h⋯O bond distance of 2.57–2.61 Å at room temperature (2.56–2.57 Å at 120 K); only for M=Mg it is increased to 2.65 Å. Any marked temperature-dependent variation of the unit-cell dimension is observed only vertical to the layers. The analysis of the infrared (IR) spectroscopy data evidences that the internal PO 4 vibrations are insensitive to the size and the electronic configuration of the M 2+ ions. The slight strengthening of the intra-molecular P–O bonds in the Mg salt is caused by the more ionic character of the Mg–O bonds. All IR spectra exhibit the characteristic “ ABC trio” for acidic salts: 2900–3180 cm −1 ( A band), 2000–2450 cm −1 ( B band) and 1550–1750 cm −1 ( C band). Both the frequency and the intensity of the A band provide an evidence that the PO 2(OH) 2 groups in M[PO 2(OH) 2] 2·2H 2O compounds form weaker hydrogen bonds as compared with other acidic salts with comparable O⋯O bond distances of about 2.60 Å. The observed shift of the O–H stretching vibrations of the water molecule in the order M=Mg>Mn≈Fe≈Co>Ni>Zn≈Cd has been discussed with respect to the influence of both the character and the strength of M↔H 2O interactions.