The compatibility, crystallization behavior, and mechanical properties of syndiotactic polystyrene (sPS)/polyester (PET)/potassium salt of sulfonated syndiotactic polystyrene (SsPS-K) were investigated. DMA results showed that all the alloys showed one T-g and the half-peak width of the sPS/PET/SsPS-K alloys became narrower compared with that of sPS/PET alloys, which decreased with an increasing content of the SsPS-K ionomer. The results of DSC showed that the T-m of sPS and PET of the alloys was similar to those of the pure materials and did not change with the content of the SsPS-K ionomer, while the initial crystallization temperature (T-0) and crystallization temperature at peak (T-p) increased. The crystallization velocity of PET increased with an increasing content of SsPS-K. The TMA results showed that the alloys could retain the perfect heat proof property of sPS. SEM micrographs showed that the addition of SsPS-K could reduce the PET domain dimension and enhance the adhesion between the PET domains and the matrix. With an increasing content of SsPS-K, the PET domain dimension was reduced continuously and dispersed more evenly. The ternary alloys had better mechanical properties and significantly higher unnotched Izod impact strength than those of the alloys without SsPS-K. When the weight ratio of sPS/PET/SsPS-K was 85/15/4, the impact strength reached a maximum of 11.5 kJ/m(2), which was about three times that of pure sPS, and still had a higher tensile strength, flexural strength, and storage modulus, which were 38.8, 54.2, and 1.55 x 10(4) MPa, respectively.