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 behavior 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.