Degradation of poly(butylene terephthalate) (PBT) induced by blending time at 260 degreesC leads to an increase of both carboxylic end groups and melt viscosity index and a decrease of the molecular weights. Crystallization kinetics of pure PET and PET blended with an epoxide, which was used to reduce the free carboxylic acid concentration, were compared. Lower carboxylic acid concentrations were obtained, but the molecular weights still decreased. The crystallization kinetics of PET blended with pigments and/or mineral fillers were also studied. Dyes, some of which are known for their nucleation activity, were used for differentiating extruded PET tubes in optical fiber loose cables. Post-shrinkage appeared to depend on the type of pigment used. Pigments concentration, particle size and chemical nature have been correlated to the PET crystallization kinetics. An increase of the nonisothermal crystallization temperature was observed, more or less important according to the pigment used. Talc and mica were also tested in PET blends, with and without pigments. Their nucleation activity parameters were measured and compared with those for the pigments. An average value of the crystallization temperature, nearly constant and independent of the pigment, was obtained with the addition of 0.15% (w/w) talc in the blends. This may be a way to obtain controlled shrinkage on industrial products with strict dimension requirements.