Intrinsic reaction coordinate (IRC) calculations of the internal rotation (torsional) potentials for H2O2 and its isotopomers HDO2 and D2O2 were carried out at the CCSD(T)/CBS//aug-cc-pVDZ level. Two extrapolation methods were used to obtain energies in the complete basis set (CBS) limit. The full IRC potential was constructed from scans from the C-2v (cis) and C-2h (trans) transition states to the equilibrium C-2 (gauche) structure. The IRC potential for H2O2 was fit to a five-term Fourier function; coefficients were compared with values obtained from spectroscopic data. The twofold IRC torsional potentials were used to obtain torsional eigenvalues, which yielded values of the transitions between various ntau states. These results compare favorably with Raman and near-infrared data. Our calculations provide values of the cis and trans barriers of 2495 and 364 cm(-1), respectively, which are in good agreement with both previously calculated and experimentally derived values. It appears that coupling between torsional motion and other degrees of freedom is not significant in these molecules. (C) 2004 American Institute of Physics.