The formation and stability of (arene)Cr(CO)3 species inside two highly porous materials-UiO-66, which is a recently synthesized metal-organic framework, and a cross-linked poly(styrene-co-divinylbenzene) resin-are investigated in detail by means of complementary spectroscopic techniques and theoretical calculations. In particular, Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis), and X-ray absorption near-edge structure (XANES) spectroscopies, coupled with theoretical calculations, allow the formation of the (arene)Cr(CO)3 species to be followed in situ, starting from the Cr(CO)6 precursor, monitoring the changes in the vibrational and electronic properties of the materials. EXAFS spectroscopy gives the structural evidence of the functionalized unities. Finally, the photoinduced reactivity of Cr(CO)3 in UiO-66 is also explored, by following the substitution of one CO ligand with a N2 molecule. The overall presented data would become the starting point for the development of a systematic procedure for investigating functionalized porous matrices.