Pluronic mixtures of F87 (E61P40E61) and F127 (E99P69E99), with E and P being poly(ethylene oxide) and poly(propylene oxide), respectively, have been used as effective separation media for the separation of small charged biomacromolecules, such as oligonulceotide fragments, by microchip electrophoresis. The temperature-dependent solubility of the middle block (P-40 or P-69) enables a convenient micelle formation of the triblock copolymer by self-assembly, forming ordered gel-like macrolattice structures at high polymer concentrations or high temperatures. Laser light scattering (LLS) and small-angle X-ray scattering (SAXS) were used to investigate the solution behavior, including the gel structure. Micelles of the F87/F127 mixture solution at different weight ratios almost had a constant hydrodynamic radius of about 10.6 nm, except for the pure F87 solution with a hydrodynamic radius size of about 6.3 nm. The aggregation number of the micelle increased from similar to 5 to similar to 17 with an increase in the F127 content in the solution. On the basis of the SAXS results, the macrolattice symmetry of the gel-like structure could be determined. Two kinds of ordered gel structures were observed: body-centered cubic (bcc, F87-rich region) and face-centered cubic (fcc, F127-rich region). There was a transition region between them whose structure had not yet been resolved. The cubic lattice constant decreased with increasing F87 content in the mixture. Under optimized conditions, oligonucleotide sizing markers ranging from 10 to 32 bases could be separated within 40 s with one-base resolution by microchip electrophoresis.