The X-ray crystal structure of the title compound, 1-methylcytosinium iodide 1-methylcytosine, [(1-MeC)2H]I (1-MeC is 1-methylcytosine) or C5H8N3O+·I−·C5H7N3O, has been determined at 163 K. In this compound, one protonated (1-MeCH+) and one neutral methylcytosine (1-MeC) moiety form an asymmetric base pair comprising three hydrogen bonds. A previous structure determination [Krüger, Bruhn & Steinborn (2004). Org. Biomol. Chem.2, 2513–2516] showed the same cell parameters but a centrosymmetric base pair in space group P21/c. At low temperature, however, we discovered the title compound to be a racemic twin showing pseudo-centrosymmetry.