The reactivity of cytosine [cyt] and 1-methylcytosine [1-Mecyt] towards transition metal ions Mn²⁺ and Co²⁺ has been tested through the analysis of their complexes. Two new compounds have been obtained from 1-Mecyt in aqueous medium, [M(H₂O)₆(1-Mecyt)₆][ClO₄]₂·H₂O [M = Mn^II1 or Co^II2], and consist of hexaaqua ions linked to six 1-Mecyt molecules in such a way as to constitute a supramolecular cationic entity. Crystallization water molecules join the supramolecular assemblies by means of hydrogen bonds. Perchlorate anions, which are held in the crystal by electrostatic interactions with the cationic assemblies, insure the electroneutrality of the compounds. Compound 2 (pale pink crystals), when not filtered, over time disappears and gives rise to dark pink crystals of formula [Co(1-Mecyt)₄][ClO₄]₂3. Compound 3 contains four 1-Mecyt molecules directly linked through N(3) atoms to Co²⁺ ions in a tetrahedral arrangement. No water molecule is present. The [Co(1-Mecyt)₄]²⁺ cationic units are joined together through hydrogen bonds and form a chain. Perchlorate anions hold these cationic chains together by means of electrostatic forces. The reaction of cyt with Mn²⁺ and Co²⁺ ions leads to compounds of formulae [M(H₂O)₄(cyt)₂][ClO₄]₂·2cyt·2H₂O [M = Mn²⁺4 or Co²⁺5] in which base pairs are formed between the two unco-ordinated cytosine molecules and those co-ordinated to the metal ion through the O(2) atom. Each metal ion is six-co-ordinated, being linked also to four water molecules. Perchlorate anions and crystallization water molecules interact with the base pairs and the metal ion through hydrogen bonds to form five-membered rings. The cobalt(II) compound constitutes the first example of cytosine–cobalt(II) co-ordination via O(2).