The number average molar mass M-n of poly(octadecene-alt-maleic anhydride) (PODMA) copolymers calculated from data obtained by size exclusion chromatography (SEC) using a polystyrene (PS) calibration was found to be inaccurate. The use of SEC combined with dilute solution viscometry enabled a method to be developed using an iterative approach, which does not require knowledge of the Mark-Houwink constants for PODMA samples. A new calibration curve was constructed as a plot of molar mass M-n for PODMA. True number-average molar masses M-n (true) calculated using the new calibration are approximately twice the apparent molar mass M-n (app) based on a PS calibration for higher molar mass samples (> 10 000 g mol(-1)).