Mixed solutes of different molecular size were separated by reciprocating size exclusion chromatography and the results were compared with conventional size exclusion chromatography with repeated feeds using computer simulation. Operational conditions, such as overlapping between elution curves and diffusivities of solutes, were varied in simulation. Simulation showed the separation efficiency was higher in RSEC than SEC with repeated feeds in the region of high diffusivity of either solutes at low degree of overlapping. Further study to improve RSEC by employing temperature swing is being made.