A novel chlorinated phosphate based on soybean oil (CPSO) was synthesized and poly(vinyl chloride) (PVC) blends plasticized with CPSO were prepared. The flame retardant properties of PVC blends were investigated with LOI tests, SEM, TGA-FTIR and TGA-MS. The results indicated that LOI value of PVC blends plasticized with CPSO reached to 30.1%. The main pyrolysis gas products of PVC blends were H2O, HCl, CO2, CO and C6H6. Phosphorous-containing components generated during the thermal degradation of CPSO, most of which were retained in the char residue rather than that in the gaseous phase. They could promote formation of char residue. The char residue was blown by evolving gases which produced a foaming and expansion of the char layer. The carbonaceous char layer is effective in protecting the underlying material from further combustion. (C) 2015 Elsevier B.V. All rights reserved.