HYDROGEN PEROXIDE DECOMPOSITION.This paper compares the catalytic activities of some transition metal ions (Fe 3+ , Co 2+ , Cu 2+ , Ni 2+ , Zn 2+ ) in the H 2 O 2 decomposition in homogenous and heterogeneous processes, including solid mixed systems (Fe-Cu-Co/Al 2 O 3 , Fe-Cu/Al 2 O 3 , Fe-Co/Al 2 O 3 and Co-Cu/Al 2 O 3 ).The solids were characterised by X-ray diffraction to explore evolution of phases or possible changes.Different trends of the catalytic activity were observed: in homogeneous medium the most active species was Fe 3+ , whereas in heterogeneous one the higher activities were shown for Co/Al 2 O 3 and Co-Cu/Al 2 O 3 .A strong cooperative effect for the Co-Cu/Al 2 O 3 system was observed, which can be considered as a new catalyst of interest for this type of reactions.