The hydrothermal synthesis and the single-crystal structure of Mg ( 1 - x ) Zn x (VOPO 4 ) 2 .4H 2 O (x ~ 0.28) are described. The structure of Mg ( 1 - x ) Zn x (VOPO 4 ) 2 .4H 2 O consists of infinite (VOPO 4 ) layers, built from VO 5 pyramids and PO 4 tetrahedra, interconnected via Mg ( 1 - x ) Zn x O 2 (H 2 O) 4 octahedra. It belongs in the triclinic space group P1 with parameters a = 6.251(4), b = 6.258(3), c = 8.065(2) 9, α = 112.85(2) o , β = 112.86(2) o , and γ = 89.93(3) o , V = 263.7(2) 9 3 , R 1 = 0.0257, and wR 2 = 0.0638 for 1985 reflections (F 2 =< 4σ(F 2 )).