The hydrothermal synthesis and the single-crystal structure of Mg(1-x)Znx(VOPO4)(2). 4H(2)O (x similar to 0.28) are described. The structure of Mg(1-x)Znx(VOPO4)(2). 4H(2)O consists of infinite (VOPO4) layers, built from VO5 pyramids and PO4 tetrahedra, interconnected via Mg(1-x)ZnxO2(H2O)(4) octahedra. It belongs in the triclinic space group P1 with parameters a = 6.251(4), b = 6.258(3), c = 8.065(2) Angstrom, alpha = 112.85(2)degrees, beta = 112.86(2)degrees and gamma = 89.93(3)degrees, V = 263.7(2) Angstrom(3), R-1 = 0.0257, and wR(2) = 0.0638 for 1985 reflections (F-2 greater than or equal to 4 sigma(F-2)).