The catalysts Pt/HZSM-5 and Pt-Cr/HZSM-5 are characterized by FTIR spectroscopy using CO as a probe molecule, and by transmission electron microscopy. The major fraction of the metal phase detected in the micrographs exhibits particles in the range of 1-3 nm uniformly distributed in the zeolite crystals. Nanodiffraction analysis of single particles confirmed the formation of fcc Pt-Cr alloy with lattice constant a = 0.386 nm. The infrared spectra at 300 K exhibit bands assigned to CO linearly bonded to zeolite-hosted metal particles at 2089 cm-1 for Pt/HZSM-5, and at 2120 and 2092 cm-1 for Pt Cr/HZSM-5. The temperature increase to 610 K resulted in a strong shift of the 2089 cm-1 band for Pt/HZSM-5 by 25 cm-1 and a slight shift of the corresponding bands for Pt-Cr/HZSM-5 by 6 cm-1. The differences are referred to different electron backdonation capacities of the CO binding Pt surface atoms of the metal particles. The lower capacity for the Pt-Cr/HZSM-5 sample is due to alloying in the metal particles.