The hydrolysis of (CH3)3Sn+, (C2H5)3Sn+ and (C3H7)3Sn+ has been studied in a Synthetic Seawater (SSW) ionic medium simulating the major composition of natural seawater, at different salinities (5≤S≤45), and at t=25 °C. Interactions with anionic components of SSW, considered as single sea salt, are determined by means of a complex formation model. By potentiometric measurements (ISE-H+ and ISE-F− electrodes), the model has been extended to also consider interactions of organotins with carbonate and fluoride ions, which are other important components of seawater. Literature and new values of hydrolysis constants in different ionic media (NaNO3, NaCl, Na2SO4 and SSW) are used to determine Pitzer interaction parameters. Predictive equations for the calculation of unknown hydrolysis constants and Pitzer interaction parameters of other trialkyltin(IV) cations, such as tributyltin(IV), are proposed.