The calibration of size exclusion chromatography (SEC) measurements for determination of the molecular weights of oligo(lactone) macromonomers was carried out by use of poly(propylene glycol) (PPG) and poly(styrene) (PS) standards. The results are compared with those obtained by end-group analysis with H-1-NMR as well as those calculated from the molar ratio of lactone and initiator in the initial mixture regarding the conversion of lactone. Clearly, calibration with PPG standards results in a better correspondence of molecular weights from SEC measurements with those of end-group analysis and calculated ones compared to calibration with PS standards.