This article describes a study on flame retardancy conducted on a dual cure thermosetting system consisting of an epoxy resin, blended with an unsaturated polyester. Neat resin panels and glass-fibre reinforced composites were prepared utilising several structurally different flame-retardant (FR) additives. These materials were tested in order to determine the Limiting Oxygen Index (LOI), which is a measure of the fire performance properties of a given material, Particular attention was paid to the FR properties of compounds, which are free of halogen such as bromine, in the absence of antimony oxide. These compounds are based on phosphorous containing molecules.