A membrane osmometer with a short response time based on a previously developed concentric design with a capillary-shaped membrane has been applied as an online SEC detector for absolute molar mass detection in combination with a concentration detector. The flow cell volume is 12.2 mu L, the response time approximately 15 s, and the molar mass cutoff is below 5 kg/mol. The performance has been tested with polystyrene standards in toluene under realistic SEC conditions.