Ab initio internal rotation barrier heights V-cis and V-trans for the peroxide form of Cl2O2 have been calculated using shape-consistent effective core potentials (ECP) and triple-zeta valence-only basis sets specially optimized for the ECP recently developed by us (Pacios, L, F.; Gomez, P. C. Int. J. Quantum Chem. 1994, 49, 817), These basis sets are augmented with standard polarization functions, and correlation is accounted for at the MPn (n = 2, 4), CCSD, and CCSD(T) levels of theory. All calculations consistently produce a cis barrier higher than the trans one, being our highest level results (CCSD(T)) : V-cis = 3538 cm(-1) and V-trans = 1890 cm(-1). This disagrees with previous experimental estimates, and the discrepancy is discussed here. Internal rotation potential and torsional constants g(beta beta)(beta) are calculated for a grid of points at the MP2 level allowing for full relaxation of the geometry, fitted to a Fourier series, and used to calculate some of the lowest torsional energy levels, A single-point calculation potential at the CCSD(T) level has also been calculated and used for this purpose. The torsional levels are presented, and the torsional fundamental transition is found to be in good agreement with the experimental values. Optimized geometry, rotational constants, harmonic frequencies, and dipole moments are presented as a test of the reliability of our calculations for future studies on other halogen peroxides.