In this work we report on hybrid organic/inorganic sol-gel based optical waveguides in slab configuration grown both on glass and silicon substrates by spin-coating process. The samples were obtained using Zr(IV)-propoxide as inorganic precursor and 3-glycidoxypropyltrimethoxisilane (GLYMO) as organic component in molar ratio of 3/1. Optical characterization of each layer has been accomplished performing refractive index measurements at different wavelengths in the visible and near infrared regions using an home-made experimental setup developed in our laboratory. Measurements have evidenced a difference between the in-plane and out-of-plane refractive index. We found that the amount of the optical anisotropy depends on the speed of spinning deposition and on the curing temperature. In particular the anisotropy increases with the increasing of speed deposition and with the increasing of curing temperature. Results were also confirmed by spectroscopic ellipsometric measurements. We attributed the optical anisotropy to a preferential orientation of the organic groups of the alkoxysilane precursor (GLYMO) during spin coating deposition.