In order to gain a better understanding of the behaviour of optical waveguides created by the implantation of helium ions into Y-cut LiNbO3, 50 keV He ion-induced lattice disorder and refractive index changes have been studied with the aid of RBS channelling and Brewster angle measurements. It is found that changes in refractive index cannot be explained in terms of lattice disorder alone and it is suggested that Li-outdiffusion plays a significant role.