The Fermi surface of Hg3- delta AsF6 is studied by means of the de Haas-van Alphen effect in magnetic fields up to 35T. The authors observe magnetic breakdown over the gaps resulting from incommensurability. The delta signal decreases in amplitude in fields larger than 10 T whereas a normal increase is seen for all other main orbits. Estimates for the effective field values belonging to magnetic breakdown and scattering are given. They set an upper bound of approximately 3 mRyd for the matrix elements of the incommensurate component of the lattice potential. A spin-orbit splitting is observed for the epsilon orbit in agreement with a self-consistent calculation including relativistic effects.