It was recently shown that the crystallization rate of poly(ethylene terephthalate) (PET) can be enhanced by introduction of diamide units either by use of comonomer during polymerization, or during a subsequent extrusion process. Here, we demonstrate an alternative solid-state chemical modification process where the amide groups are introduced into commercial PET by exposing the PET pellets simply to vapors of ethylene diamine at 125 degreesC, i.e. far below the melting point of PET. We make up for the corresponding decrease in molecular weight by a subsequent solid-state polymerization. We find that the crystallization rate enhancement is at least comparable to that achieved by using talc as nucleating agent.