The present invention concerns a sizing composition for glass fibers comprising the following components: (a) A silane based coupling agent which is not an aminosilane; (b) A film former; (c) A borate; (d) A lubricant Characterized in that, at least 75 wt. % of the silane coupling agent present in the composition is dialkoxylated. It also concerns a glass fiber sized with the reaction product of said sizing composition, as well as a polymeric composite reinforced with such glass fibers.