The reactions of one, two, or four water molecules with ethylene radical cation and the reaction of one water molecule with trans-2-butene radical cation have been investigated by ab initio calculations at the UMP2/631G* level. in part, PMP4/6-311+G** single point energies have been added. The reaction of one water molecule with ethylene radical cation occurs without barrier in the gas phase to form a distonic radical cation. This intermediate species reacts readily with a second water molecule to interchange water molecules through two S(N)2-type reaction pathways. The influence of solvation on the substitution barrier has been studied in various ways and has been shown to increase the gas phase substitution barrier approximately twofold in water. The thermodynamic acidity of the intermediate distonic radical cation has been estimated using a thermodynamic cycle. From combined ab initio and solution simulation results, the acidity of the distonic ion is predicted to be somewhat smaller as compared to that of protonated ethanol. The reaction of trans-2-butene with water in the gas phase does not lead to the formation of a distonic radical cation. Instead, an ion-dipole complex is formed. This result is linked to the stabilization of alkene radical cations by alkyl substituents.