The geminal complexes formed after adsorption of small molecules on cationic sites may be divided into two main groups: site- and complex-specified. Complex-specified formation always occurs during the interaction of definite cations with certain molecules. Typical examples are the Co2+(NO)(2) species. A peculiarity, in this case, is that the geminal complexes are more stable than the respective linear species. The site-specified formation of geminal complexes is determined by the low co-ordination number of the cation serving as adsorption centre. Typical examples are the geminal species formed on metal cations exchanged in ZSM-5 zeolite: Cu(CO)(2) and Na+(N-2)(2). In this case, the linear complexes are more stable than the geminal species. Mathematical expressions describing the adsorption isotherms as well as the ratios between the linear and geminal species at different coverages are proposed. The co-adsorption of two gases is also considered. Simultaneous adsorption of three molecules on one centre is illustrated by the formation of Cu+(H2O)(2)CO adducts on Cu-ZSM-5. Finally, the possibility of formation of species that are site- and complex- specified is discussed.