Although OClO photolysis was one of the first photochemical reactions to be studied, the primary photochemistry of OClO still remains controversial. Several studies have demonstrated that Cl2O4 is one of the major photolysis products of OClO, in addition to ClO+O and ClOO. Thus there is a need to analyze the chemical properties of Cl2O4 and its related species. In the present study we have investigated the equilibrium structures and harmonic vibrational frequencies of chlorine perchlorate (Cl2O4), fluorine perchlorate (FOClO3), and hydrogen perchlorate (HOClO3) at Hartree-Fock and MP2 levels using 6-31G* basis set. Normal mode analysis has been carried out using ab initio derived force fields at the correlation level for the 6-31G* basis set. The calculated frequencies of XOClO3 (X=Cl, F, and H) molecules are discussed in comparison with those determined from experiment and with the corresponding vibrations of Cl2O7. In addition, the paper draws attention to the possible role of Cl2O4 formation in the polar stratosphere with relevance to ozone layer depletion.¿ 1997 American Geophysical Union