At disordered Si/SiO2 interfaces the lattice mismatching results in dangling bond Pb centres forming a rather broad distribution of energy states. In this study these energy distributions have been determined using isothermal current Laplace deep level transient spectroscopy (DLTS) for the (100) and (111) interface orientations. The (111) distribution is 0.08 eV broad and centred at 0.38 eV below the silicon conduction band. This is consistent with only Pb0 states being present. While for the (100) orientation this distribution is broader (0.1 eV) and deeper (0.43 eV) on the energy scale. Detailed studies revealed two types of the interface states in this broad distribution: one similar to the (111) orientation while the other has a negative-U character in which the emission rate versus surface potential dependence is qualitatively different from that observed for Pb0 and is presumed to be Pb1. Discrepancies between Pb states energy distributions obtained with a use of the isothermal Laplace and conventional DLTS measurements are discussed. The presented experimental procedure can be used for analysis of interface states observed at interfaces of other semiconductor-oxide/dielectric systems.