Carbon monoxide and dinitrogen adsorbed, at nominally liquid-nitrogen temperature, on Mg2+-exchanged ETS-10 were found to form Mg2+... CO and Mg2+... N-2 adducts involving cations located in the main channels (12-membered rings) of the titanosilicate molecular sieve. These adducts gave main IR absorption bands at 2190 and 2203 cm(-1) for CO, and at 2336 and 2339 cm(-1) for N-2, which were assigned to the fundamental stretching mode of the diatomic molecules polarized by the electric field created by Mg2+ ions. Corresponding adducts with Na+ and K+ ions, not exchanged with Mg2+, were also present. The results, which are relevant to the potential use of ETS-10 as a catalyst, lend support to previous structural studies suggesting the existence of two different cation sites along the main channel of ETS-10.