INFLUENCE OF THE MEDIUM RIGIDITY ON THE PHOTOCHROMISM KINETICS OF METAL DITHIZONATES. The kinetics of photoisomerization and thermal back reaction of mercury(II) and zinc(II) dithizonates, M(HDz) 2 , were analyzed by photolysis assays in fluid solution and in poly(methyl methacrylate), PMMA, films. In fluid solution, the photo-and thermoisomerization of both complexes can be described by a single exponential function. The Zn(II) dithizonate exhibits rate constants three orders of magnitude greater than that for Hg(II) complex, which is attributed to the higher charge density of the Zn(II) cation. In PMMA films, the thermal back reaction of both complexes can only be described by bi-exponential functions. On this more rigid medium the rate constants are about one order of magnitude slower than the respective ones in fluid solution and the bi-exponential behavior is related to regions in the polymeric matrix with distinct free volumes. The rate constants for both photo-and thermoisomerization of Zn(HDz) 2 in PMMA are reported for the first time and are compared with the well-studied Hg(II) analog. The experimental data provides insights on the role of the metal cation on the photocromic behavior in rigid media, which can lately contribute for technological applications.