The ability of Octa Decyl Thiol (ODT) compound in passivating Gallium Arsenide (GaAs) surfaces is studied by comparing the XPS spectra on the ODT treated and untreated GaAs samples and by comparing the electrical characteristics of Schottky diodes and MIS capacitors fabricated on them. A detailed study on MIS capacitors and an evaluation of the interface state density (D-it) has clearly demonstrated that even though the ODT treatment reduces the interface state density in the upper half of the band gap more than an order of magnitude compared to untreated samples, it is not effective in reducing the midgap D-it values. From the XPS spectra taken on the ODT treated GaAs surfaces, this is attributed to an island-like ODT coatings and to the inability of this sulphur compound treatment to remove the native oxide present on the GaAs surface.