Abstract The molecular weight evaluation of poly(ethylene terephthalate) (PET) from different sources was accomplished by using three different techniques: solution intrinsic viscosity, intrinsic viscosity from melt flow index (MFI) and size exclusion chromatography (SEC). Five PET samples were used: commercial virgin PET, designed for bottle production (BPET); virgin PET taken from the first stage of condensation polymerization, similar to fiber grade PET (FPET); PET flakes from post-consumer soft drink bottles (RPET); RPET composite containing 30 wt% of fiberglass (RPETComp) and RYNITE®, a commercial PET composite containing 30 wt% of fiberglass. Solution intrinsic viscosity was determined by three methods: Billmeyer's equation (ASTM D 4603), successive dilutions (ASTM D 2857) and Schulz-Blaschke's equation. Intrinsic viscosity from MFI was obtained with the aid of melt flow index equipment provided with an internal calibration curve which allows the direct correlation between melt flow index (MFI) and intrinsic viscosity. Size exclusion chromatography (SEC) was carried out in hexafluoroisopropanol (HFIP) with PMMA standards, which allowed the determination of Mn ¯ , Mw ¯ and polydispersity of the PET samples. Molecular weights were also calculated by Mark–Houwink's equation, with values of K and a taken from the literature. Correlation between molecular weight and intrinsic viscosity values obtained by different methods was done as an attempt to estimate the molecular weight of an unknown PET sample from its known intrinsic viscosity.