Thiophene, thiophene-2,5-d₂ and thiophene-d₄ adsorption on CaO activated at 723-973 K has been investigated at 170-300 K by means of IR spectroscopy and IR analysis of desorbed gas. Molecular adsorption of thiophene occurs on surface oxygen ions and basic OH groups by weak H-bonding of CH groups with the lone electron pairs of oxygen. On the most basic oxygen ions of CaO activated at 973 K, thiophene dissociates already at 200 K with the formation of surface OH groups and thiophenide C₄H₃S^- anions. This reversible dissociation accounts for the catalytic reaction of isotopic scrambling of thiophene-2,5-d₂ that takes place already at 220 K and leads to the appearance of all 10 possible thiophene isotopomers in the desorbed gas. Experiments with thiophene coadsorption with CO enable one to associate the sites of thiophene dissociation with those oxygen ions that bound CO giving carbonite CO₂^2- ions. Three kinds of such sites could be distinguished, which differ in the band positions of the resulting OH groups and in thermal stability of corresponding thiophenide anions.