The invention provides a process for improving the fire resistance of a thermoplastic or thermoset organic polymer composition, characterised in that an alkoxysilane containing at least one organic nitrogen-containing group and an alkoxysilane or silicone resin containing at least one group selected from phosphonate and phosphinate groups are added to a thermoplastic or thermosetting organic polymer composition and heated in the presence of moisture to cause hydrolysis and siloxane condensation of the alkoxysilane or alkoxysilanes. The alkoxysilanes, or alkoxysilane(s) and silicone resin, of the invention are particularly effective in increasing the fire resistance of polycarbonates and blends of polycarbonate with other resins such as polycarbonate/ABS blends.