Novel mixed valence compounds with the formula α-[Cu 2 X(C 5 H 3 N 2 O 2 ) 2 (H 2 O)] n [X=Cl(1), Br(3), C 5 H 3 N 2 O 2 =2-pyrazinecarboxylate] and their isomers β-[Cu 2 X(C 5 H 3 N 2 O 2 ) 2 (H 2 O)] n [X=Cl(2), Br(4)] have been synthesized and structurally characterized. Crystal data: 1, orthorhombic, Pbca, a=12.8040(7), b=7.7323(4), c=26.104(1) Å, V=2584.4(2) Å 3 , Z=8, R1=0.0559; 2, triclinic, P1, a=6.1908(7), b=7.7246(8), c=13.968(2) Å, α=78.853(2), β=81.455(2), γ=78.036(2)°, V=637.0(1) Å 3 , Z=2, R1=0.0592; 3, orthorhombic, Pbca, a=12.7082(6), b=7.7676(4), c=26.1548(12) Å, V=2581.8(2) Å 3 , Z=8, R1=0.0257; 4, triclinic, P1, a=6.2462(5), b=7.7568(6), c=14.1121(10) Å, α=78.082(1), β=81.702(1), γ=79.096(1)°, V=652.97(9) Å 3 , Z=2, R1=0.0290. The α-isomers 1 and 3 have two-dimensional layer structures which are composed of mixed valence chains of [Cu I Cu II (C 5 H 3 N 2 O 2 ) 2 (H 2 O)] and chains of corner-sharing {Cu I N 2 X 2 } tetrahedra. The β-isomers 2 and 4, however, have three-dimensional structures with the [Cu I Cu II (C 5 H 3 N 2 O 2 ) 2 ] chains interconnected by an alternative arrangement of {Cu II N 2 O 4 } octahedra, {Cu I N 2 OX} tetrahedra and {Cu II N 2 O 2 X 2 } octahedra. The spectroscopic and magnetic properties have been investigated.