SSP, starting from very low molecular weight (MW) poly(ethylene terephthalate) (PET) precursors, is claimed to offer significant production cost advantages over conventional PET production. However, as the intrinsic viscosity (IV) of the PET precursor is reduced, there is a significant change in the crystallization behavior of PET and morphology that affects reactivity in SSP. Using small particle size PET to significantly reduce the effects of diffusion so that SSP is under chemical reaction control and using a kinetic model that describes an overall SSP rate, the effect of ethylene isophthalate substitution on the SSP rate from low MW PET precursor was determined. As the ethylene isophthalate comonomer content increases, the rate of SSP for low MW PET increases. The activation energy for SSP of low MW PET decreases with an increase in the ethylene isophthalate content. For the low MW PET copolyesters in this study, the SSP activation energy is comparable to conventional process when the comonomer content of the low MW polyester is around 7 mol % and the conventional precursor is around 3 mol %. However, even though the activation energy is reduced through the use of higher comonomer content, the overall rate of SSP for the low MW copolyesters studied is significantly slower than conventional SS. This reduction in rate is explained by differences in crystallinity.