Phosphate-polyester copolymers were synthesized from spirocyclic pentaerythritol di(phosphate acid monochloride)s (SPDPC), which is an intumescent, phosphate flame retardant. Because of the new monomer being introduced, the structure and properties of the polyester were altered. The effects of the phosphate on the properties of flame retardance oxygen index, (OI), glass-transition temperature(T-g), and solubility parameters (delta) were studied. It was found that the OI, T-g, and delta increase with increasing SPDPC content in the copolymers. The flame-retardant mechanism was found to possibly be the solid phase intumescence found by analysis of the SEM photos of the copolymer chars.