In a continuing search for mixed-valence copper phosphate compounds, crystals of a novel copper(I/II) oxophosphate chloride, Na2Cu4P2O9Cl, have been grown in a low-temperature eutectic flux of 23% NaCl and 77% CuCl (mp = 315 degrees C). The title compound crystallizes in an orthorhombic lattice, Cmcm (No. 63), with a = 13.614 (2) Angstrom, b = 10.385 (2) Angstrom, = 6.372 (2) Angstrom, and V = 900.8 Angstrom(3); Z = 4. This new compound is a welcome addition to the small family of electronically distinct copper phosphates. The extended structure exhibits an interesting quasi-one-dimensional chain consisting of fused tetrahedra of copper(II) atoms centered by a mu(4)-oxo-bridging oxygen atom. This mixed framework is composed of [Cu(2)O-II(6)]-[Cu(1)(O4Cl)-O-II]-[Cu(3)(O2Cl3)-O-I] fused polyhedra. The Cu2+ centers are closely spaced, 3.03-3.19 Angstrom, creating an environment conducive to a magnetic transition at similar to 5 K associated with the diffused antiferromagnetic coupling starting at 220 K. In this paper, the synthesis, thermal properties, and bond valence analyses of Na2Cu4P2O9Cl are also addressed.