The potential energy surface of disiloxane, SiH3OSiH3, has been investigated in quantum-mechanical ab initio calculations at the SCF, MP2-MP4, QCISD(T), and CCSD(T) levels of theory, using the correlation-consistent polarized cc-pVXZ basis sets. The total energy of disiloxane was calculated as a function of coordinates of the three large-amplitude motions : the SiOSi bending and internal rotations of the silyl groups. The structural parameters were optimized at the MP2 level, and the optimum parameters were determined for several values of each of the coordinates of the large-amplitude motions. The equilibrium potential energy surface of the large-amplitude motions was thus determined. Using extensively correlated wave functions, the potential energy surface is calculated to be very anharmonic, with a barrier of approximate to 130 cm(-1) at the linear configuration of the SiOSi skeleton and a minimum at the SiOSi angle of approximate to 147 degrees. The 3-fold torsional barrier is determined to be very small, indicating nearly free internal rotation of the silyl groups. The theoretical results are in good agreement with experimental data and consistent with the quasi-symmetric top model of the disiloxane molecule.