A synergistic effect on flame retardancy, thermal stability, and mechanical properties was found when sepiolite was incorporated into a linear low-density polyethylene (LLDPE)/magnesium hydroxide (MH) composite. Different amounts of sepiolite (up to a maximum concentration of 15 phr) were added to a standard LLDPE/MH formulation, and vinyltriethoxysilane was used as a compatibilizer as well as a crosslinking agent. The thermal stability and the oxidation induction time increased with increasing sepiolite content in the LLDPE composites. Limiting oxygen index (LOI) results indicated an increase in LOI with the addition of sepiolite, and an LOI value of 36.5% was observed with 15 phr sepiolite in the LLDPE/MH formulation. The addition of sepiolite increased the gel content and tensile strength of all samples and lowered the elongation at break. The heat deflection, Vicat softening temperature, and hardness were also improved by the incorporation of sepiolite. This synergistic behavior of sepiolite with MH could be used in halogen-free, flame-retardant LLDPE formulations. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2772-2777, 2011