The glass transition temperature (T-g), crystallization, and melting character of a class of random polyester ionomers (polymer containing < 10 mol % ionic groups) were investigated. The nonlinear change of the T-g and crystallization and melting behavior were characterized using differential scanning colorimetry (DSC). The ionomers are derived from polyethylene terephathalate (PET) modified through copolycondensation with a fully neutralized sulfonate moiety (sodiosulfo) isophthalate (Na-SIP). Significant and systematic changes in the glass transition temperature and thermal characteristics upon addition of Na-SIP on the PET backbone were observed, indicating strong association and interaction on the ionic species. At Na-SIP levels greater than or equal to 4 mol %, the turn of the the glass transition temperature was found, and the same results were obtained for the samples treated either by quenching or dissolution, suggesting the presence of reversible crosslink and aggregation of the ionic species within the organic matrix. When crystallized from the healing or cooling the samples during the DSC nonisothermal crystallization run at a 10degreesC/min, the enthalpy of the cold crystallization and melting showed an obvious decrease with the increase of Na-SIP content, and changes of the crystal temperature had an analogy to those of the T-g. A tune of the crystal temperature was found at Na-SIP levels greater than or equal to 3 mol % (see Figs. 4, 5, and 7). The experimental data were discussed in the context of the restricted mobility model of the aggregation in the ionomers.