A type of lanthanide (La(III))-containing ionomer based on acrylate processing aid (ACR) for poly (vinyl chloride) (PVC) was synthesized, and influence of the ionomer on thermal stabilization of PVC was investigated with visual color comparison and Congo red methods. Results revealed that the ionomer with a suitable La(III) content behaved as a good costabilizer to PVC. It was able to extend static stabilization time of PVC and postpone zinc burning. The stabilizing efficiency of the ionomer to PVC depended on ion content, which was discussed in terms of EisenbergHirdMoore model. Moreover, Fourier transform infrared test verified that this ionomer can react with zinc stearate (ZnSt2) to form some new structures, which is responsible for postponing zinc burning. The ionomer and epoxidized soybean oil exhibited a synergistic effect on the stabilizing efficiency of calcium stearate (CaSt2)/ZnSt2 stabilizer to PVC compounds. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012