[en] The monoclinic crystal form of human peroxiredoxin 5 with eight molecules in the asymmetric unit was obtained under exactly the same conditions as the tetragonal form with one molecule in the asymmetric unit, except that the latter was briefly cryosoaked with halide for derivatization. A merohedral twinning was observed, which is rather unusual in the monoclinic system and only possible with particular unit-cell dimensions. After detwinning the native and a mercury derivative, the structure was solved by the SIR method with the help of the non-crystallographic symmetry. The packing of the monoclinic and tetragonal forms are compared, with special attention to the role of bromide ions in the change of space group after crystallization. The availability of nine (eight monoclinic plus one tetragonal) independent molecules allows an analysis of the mobility. The two Cys residues implicated in the peroxide-reduction mechanism are located in rigid regions but are covered by mobile loops.