As has been proven earlier, ruthenium-containing NaHY zeolites are able to catalyze the decomposition of ammonia at temperatures from 300 to 450 degrees C. In such catalysts, ruthenium cations are still present, even after heat treatment in high vacuum at 400 degrees C; they can be detected using ammonia and/or pyridine as probes for Fourier transform IR spectroscopy. They reside both in supercages and in sodalite cages. Various intermediates of the decomposition of the Ru(NH3)(6)NaY complex on heat treatment in high vacuum were identified via in situ IR spectroscopy; in particular, evidence for the formation of complexes with nitrosyl ligands was obtained. It was shown that partially decomposed (deammoniated) Ru(NH3)(6)NaY complexes can be recovered to some extent by readsorption of ammonia. Ruthenium-containing species were localized either in the supercages or in the small cavities as shown by LR spectroscopy employing ammonia and pyridine as probes, The acidic properties of variously treated Ru(NH3)(6)NaY zeolites were characterized via temperature-programmed desorption (TPD) of ammonia, which was monitored by mass spectrometry. A strong interaction between ruthenium-containing species and the zeolite framework, leading to a lack of overtone and combination modes in the near infrared, is confirmed. Investigations of Ru(NH3)(6)NaY samples by X-ray photoelectron spectroscopy under the same conditions as applied for IR and TPD studies revealed that, at variance with the results usually obtained after heat treatment of Ru(NH3)(6)NaY in high vacuum, no significant formation of ruthenium metal species through autoreduction occurred. Rather, a particular form of a cation-exchanged Ru, Na-Y zeolite was obtained.