The tin(II) phosphate, [N(C2H5NH3)(3)](3+) [Sn(PO4)(HPO4)](3-) 4H(2)O, contains monomeric Sn-2(PO4)(2)(HPO4)(2) units which form a hydrogen-bonded network with the TREN amine (TREN = N(C2H5NH3)(3)(3+)); triclinic, space group P1 (no. 2), a = 9.579(1), b = 10.507(1), c = 10.976(1) Angstrom; alpha = 72.93(1)degrees, beta = 78.03(1)degrees, gamma = 69.82(1)degrees, V = 984.2(1) Angstrom(3), Z = 2, The Sn2P2O4 core of this unit is a common structural feature of open-framework tin phosphates and may play a role as a building block for this class of materials. (C) 1998 Academic Press.