A double polyphosphate of ammonium and erbium NH4Er(PO3)(4) (type IV) has been prepared for the first time. It crystallizes in a monoclinic system and its parameters are a = 11.02(2) Angstrom, b = 7.70(1) Angstrom, c = 10.39(2) Angstrom, beta = 107.7(1)degrees, V = 839.9 Angstrom(3), d(x) = 3.14, Z = 4. The corresponding space group is P2(1)/n. This compound has been identified as a chain structure by infrared spectra. A thermal analysis study of two condensed polyphosphates NH4Er(PO3)(4)-IV and HEr(PO3)(4) is presented; TG and DTA curves are given.