This paper describes a detailed study on effects of moisture and solid phase polymerization on melt viscoelastic properties of poly (ethylene terephthalate) (PET). We found a drying condition and an experimental procedure which did not induce any variation of either molecular weight or molecular weight distribution in samples during periods of sample preparation and a short time run of viscoelastic measurements. For a long time run, η′ and G′ increased with time and especially the increase of the latter was remarkable. The similar behaviour was observed for nylon-66 but not for poly (butylene terephthalate) (PBT). It is suggested that highly branched and/or cross-linked substances were produced in the samples besides linearly polymerized ones.