Acrylate processing aid (ACR)-based La(III)-neutralized ionomers with different acid and La(III) ion contents were synthesized. Increasing lanthanum content in the ionomers led to an increment in the size of the ionic aggregate. Increasing acid content resulted in an increasing glass transition temperature ascribed to ion aggregation and to the occurrence of dense hydrogen bonding. The influences of acid and ion contents of the ionomers on the fusion and thermal stabilization behaviors of poly(vinyl chloride) (PVC) were investigated. The results revealed that at the low acid content of 0.25 mmol/g of hydrolyzed ACR, the stabilization efficiency of the ionomers enhanced with an increase of ion content before the appearance of clusters. At acid contents of 1.80 and 2.90 mmol/g, the ionomers exhibited poor ability to stabilize PVC during processing. The fusion-promoting efficiency of the ionomers showed the same trend as the stabilization efficiency. Cluster formation and increasing acid content were also disadvantageous for ionomer use in promoting PVC fusion. J. VINYL ADDIT. TECHNOL., 19:86–93, 2013. © 2013 Society of Plastics Engineers