AbstractFormation constants for complex species of mono‐, di‐ tri‐alkyltin(IV) cations with some nucleotide 5′‐monophosphates (AMP, UMP, IMP GMP) are reported, at T = 25 °C at I = 0.16 mol l−1 (NaCl). The investigation was performed in the light of speciation of organometallic compounds in natural fluids in the presence of nucleotides whose biological importance is well recognized. The simple mixed hydrolytic complex species formed in all the systems investigated in the pH range 3–9 are (L = nucleotide; M = organotin cation Rx Sn(4−x)+, with x = 1 to 3): ML+, ML(OH)0 ML(OH)2− for the system CH3Sn3+–L (L = AMP, IMP, UMP); ML0 ML(OH)− for the system (C2H5)2Sn2+–L (L = AMP, IMP, UMP); ML−, ML(OH)2−, MLH0 M2L(OH)− for the system (C2H5)3Sn+–L (L = AMP); ML, ML(OH)2− M2L0 for the system (C2H5)3Sn+–L (L = IMP, UMP). As expected, owing to the strong tendency of organotin(IV) cations to hydrolysis in aqueous solution, the main species formed in the pH range of interest of natural fluids are the mixed hydrolytic ones. Copyright © 2004 John Wiley & Sons, Ltd.