Photolysis at 6 K of OClO trapped in a single crystal of KClO4 produces the isomer ClOO in three inequivalent configurations A, B, and C. A and B are metastable, the conversions A --> B and B --> C occurring at similar to 35 and similar to 60 K, respectively. Comparison of observed values of the O-O stretching frequency nu(1) with those reported for ClOO matrix-isolated in argon and the geometry deduced from the spin Hamiltonians both suggest that ClOO in the stable configuration C is only slightly perturbed by the KClO4 lattice while stronger perturbations occur in configurations A and B. Hence, the spin Hamiltonian obtained for C should represent the properties of the free ClOO molecule to a good approximation. The matrix effects observed for A and B are attributed to polarization of ClOO, transferring electronic charge from Cl to O-2.