In chromatography, a solute partitions between a mobile and a stationary phase. Uniquely in size-exclusion chromatography, both phases have identical physicochemical properties; differential partition is effected by restricting the access of the solute to the stationary phase. At its simplest, the stationary phase is held inside pores with a distribution of sizes, and some pores are too small to permit the entry of larger molecules-hence size exclulsion (SE) (or the alternative gel permeation) chromatography.