2-Hy-droxy-methyl-1,3-dimethyl-1H-benzimidazol-3-ium iodide.

In the cation of the title compound, C10H13N2O+·I−, all non-H atoms, with the exception of the O atom, are essentially coplanar, with a maximum deviation of 0.04 (1) A. In the crystal, the cations and anions are arranged in layers parallel to (100). The cations are connected to the anions via an O—H⋯I hydrogen bond and there are significant π–π stacking inter­actions between cation layers, with centroid–centroid distances in the range 3.606 (5)–3.630 (5) A. A weak intra­molecular C—H⋯O hydrogen bond is also observed. The crystal studied was an inversion twin with refined components of 0.52 (5) and 0.48 (5).