Abstract: The invention relates to environmentally friendly flame retardant nucleating compositions, and corresponding formulations and processes useful for the production of polystyrene foams with low densities and a very low halogen content in the final product. The flame retardant composition according to the present invention achieves good flame retardancy and excellent recycling performances, with about the same loading of traditional halogenated compound, which does not affect the process and the physical and mechanical properties of the final foam and can be considered halogen free, according to some international standards.