A review with 17 refs. (including Authors' own works) covering modifications of the structure of hydrotalcites (HT) by introducing transition metal cations (Cu2+, Co2+, Ni2+, Fe3+) into the brucite-like sheets or by intercalating polyoxometallate anions (V10O286-, Cr2O72-, Mo7O246-, W7O246-) into the interlayer space to replace the interlayer anions. The calcined hydrotalcites, BET surface 170-270 m(2)/g, were used to catalyze the decomposition of N2O. The transition metal oxide dispersed in the MgO-Al2O3 matrix affected the catalyst activity in the descending order. Cu > Co > Fe > Ni. In the presence of water vapor or oxygen, the most active catalyst, HT-Cu, was deactivated, and after removal of H2O or O-2, slowly reactivated.