The crystal structure of manganese(III) dihydrogen triphosphate dihydrate, MnH2P3O10 · 2H2O, has been studied by means of X-ray powder diffraction data using the Rietveld method. The compound crystallizes in the monoclinic space group P2/c, with unit cell dimensions a = 7.8286(2), b = 4.9503(2), c = 12.25415(18) Å, β = 95.7922(11)°, V = 472.47(2) Å3, Z = 2. It has a layered structure, with manganese-triphosphate polymeric layers, parallel to the bc plane, and water molecules located in the interlayer space. Powder pattern of the compound exhibits anisotropic peak broadening, which cannot be explained by crystallite size effects. The reflections with k = 2n remain narrow, whereas those with k = 2n + 1 and h ≠ 0 are widened substantially. These features have been assigned to stacking faults with a b/2 translation. Simulated diffraction patterns, calculated with the program DIFFaX by recursion method for different values of stacking faults probability, reproduced the experimental X-ray powder pattern. A phenomenological treatment of anisotropic peak broadening was used to complete the Rietveld refinement.