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The thionucleoside 2-thiocytidine (C2S) occurs in nature in transfer RNAs; it receives attention in diverse fields like drug research and nanotechnology. By potentiometric pH titrations we measured the acidity constants of H(C2S) + + (pK a = 4.24). This moderate effect of about 0.8 pK units contrasts with the strong acidification of about 4 pK units of the (C4)NH 2 group in C2S (pK a = 12.65) compared with Cyd (pK a & 16.7); the reason for this result is that the amino-thione tautomer, which dominates for the neutral C2S molecule, is transformed upon deprotonation into the imino-thioate form with the negative charge largely located on the sulfur. In the M(C2S) 2+ complexes the (C2)S group is the primary binding site rather than N3 as is the case in the M(Cyd) 2+ complexes, though owing to chelate formation N3 is to some extent still involved in metal ion binding. Similarly, in the Zn(C2S-H) + and Cd(C2S-H) + complexes the main metal ion binding site is the (C2)S -unit (formation degree above 99.99% compared with that of N3). However, again a large degree of chelate formation with N3 must be surmised for the M(C2S-H) + species in accord with previous solid-state studies of related ligands. Upon metal ion binding, the deprotonation of the (C4)NH 2 group (pK a = 12.65) is dramatically acidified (pK a & 3), confirming the very high stability of the M(C2S-H) + complexes. To conclude, the hydrogen-bonding and metal ion complex forming capabilities of C2S differ strongly from those of its parent Cyd; this must have consequences for the properties of those RNAs which contain this thionucleoside.Keywords Acidity constants Á Isomeric equilibria Á RNAs with thionucleosides Á Stability constants Á Tautomeric equilibria Species written without a charge either do not carry one or represent the species in general (i.e., independent of their protonation degree); which of the two possibilities applies is always clear from the context. A formula like (C2S-H) + means that the ligand has lost a proton and it is to be read as C2S minus H + .