A versatile approach for the modification of polybutylene terephthalate (PBT) surface properties without using solvent, through molten chemical modification of PBT hydroxyl end groups, with n-octyltriethoxysilane is reported. After optimization of the reactional conditions, thermally stable hydrophobic modified PBT was successfully synthesized at 240 degrees C with 0.3 and 1 wt. % of reagent by reactive extrusion. With the addition of 1 wt. % of n-octyltriethoxysilane, the water contact angle value increased from 80 +/- 2 degrees for raw PBT to 115 +/- 4 degrees for the modified one. Furthermore, water transport behaviors were also investigated. The modified PBT with 1 wt. % of reagent depicted a decrease of water diffusion coefficients from 1.58 x 10(-10) for reference PBT to 0.64 x 10(-10) cm(2).s(-1) for the modified one. The water permeability coefficients reduced also from 8.3 for reference PBT to 1.5 barrer for the modified PBT.