The density of aqueous two-phase systems obtained from water, dextran and poly(ethylene glycol) (PEG) was increased by addition of metrizamide, which had a greater affinity for the upper (PEG-rich) phase than for the lower (dextran-rich) phase. The density of both phases could be increased to 1.2 g ml −1. Above certain concentrations of metrizamide the dextran phase became the lighter one. Isopycnic phases were obtained by careful adjustment of the composition of the system, and the effect of metrizamide on phase composition was characterized. The systems were tested by partition of starch grains. Fractionation of mature pollen grains and starch grains was achieved by a counter-current distribution process using these systems.