We describe the synthesis and refractive index characterisation of four organo-siloxanes materials, applicable to the fabrication of photo-patternable channel waveguides. By combining aliphatic, aromatic and vinylic alkoxysilane precursors, organically modified silicates materials have been synthesised using the sol-gel process. Control of the refractive index was achieved through the variation of aliphatic and aromatic alkoxide content within the materials. It is clearly demonstrated that the presence of aromatic groups significantly increases the refractive index to 1.562. Conversely, the addition of vinylic groups decreases the value to 1.490. These findings are attributed to the atypical polarisability exhibited by aromatic and aliphatic precursors, which strongly affect the overall molar refraction. (c) 2006 Elsevier B.V. All rights reserved.