Unsaturated polyester resins having improved fire retardant properties comprise a halogen-containing unsaturated polyester of a polycarboxylic compound and a polyhydric alcohol together with an additive mixture comprising iron or a compound of iron and at least one member of the group consisting of copper, a compound of copper, antimony and a compound of antimony. The unsaturated polyester resin may be copolymerized with an unsaturated monomer to prepare castings, laminated products, reinforced articles and the like having low burning rates, low smoke generation, and beneficial char formation in the presence of a flame.