We have examined the influence of the solvent mixture diphenyl ether-biphenyl (DPE-BP) on the post-polymerization of poly(ethylene terephthalate) (PET) of intrinsic viscosity (IV= 0.42 dL/g) in the swollen state and in solution, by following the IV increase and the end-group depletion. During the swollen state polymerization (SwSP) of thin disks (180 mu m) at 195 degrees C, the initially rapid step growth polymerization slows dramatically beyond IV= 1.2 dL/g in 5 h, and is unable to proceed beyond 1.4 dL/g. This appears to be related to the temporarily restricted mobility of the end groups due to the observed solvent induced crystallization, because sufficient reactive end groups can be directly detected, and further post-polymerization in melt state is possible. When limitations due to crystallization are eliminated by carrying out post-polymerization in solution at 250 degrees C, it proceeds to IV= 1.8 dL/g in a single step. Since solution polymerization eliminates the requirement of handling fine PET particles, it offers an attractive route to high molecular weight PET, particularly when the solution can be directly used for further processing, e.g. into fibers. (c) 2005 Published by Elsevier Ltd.