This work presents a study performed on several Au contacts deposited by evaporation on oxide free and oxidised (5-20Angstrom of oxide) a-Si:H surfaces. The characterisation of the films was performed on as deposited, aged and annealed at 150degreesC structures. SIMS and RBS measurements show that the Au diffuses very easily on oxide free a-Si:H surfaces, even at room temperature, resulting in the formation of an oxide at the device surface that acquires a blue col our instead of the gold colour of the contacts. This was also visible in the SEM pictures of the cross section of the structures produced and on the changes of the surface morphology observed by AFM measurements. On the other hand, when the Au is deposited on oxidised a-Si:H surfaces, the results show that the oxide prevents the Au from diffusing and the nature of the contact is preserved. That is, better rectifying and stability performances are obtained in MIS like structures than in Schottky structures.