Gd3+-ESR spectroscopy can be used as a sensitive method for the study of lanthanide additives in catalysts. Here we present the results of a comparative study of Gd/SiO2-Al2O3 and Gd/HZSM-5. ESR gives evidence of rigid bonding of isolated Gd3+ ions into both amorphous silica-alumina and into HZSM-5. In addition, the zeolitic matrix stabilizes very small Gd3+-clusters (containing only a few ions) capable of interacting with water molecules. Excess Gd is present as non-dispersed, particulate oxide. Strong bonding of PO43- anionic ligands irreversibly changes the local environment and reactivity towards H2O of the Gd3+-clusters in HZSM-5. The Gd3+ ions do not block the cationic positions of HZSM-5 from further interaction with paramagnetic CU2+ or Rh2+ cations.