The courses of ion-molecular reactions in gaseous phase and in solutions significantly differ. From the aspects of classical chemical conceptions, mechanisms of these reactions in gaseous phase are unusual. This has been demonstrated for the reactions of hydrogen halides (HX, X = F, Cl, ... At) with their radical cations (HX^+.) and for the reactions of methyl halides with their radical cations (CH₃X + CH₃X^+.). The courses of these processes were investigated by the methods of quantum chemistry; experimental data are at disposal in literature for some of the processes. (in Czech)