The atypical lower critical ordering transition (LCOT) behavior has been revealed for poly(ethylene oxide)-block-poly(2-vinylpyridine) (PEO-b-P2VP) by means of temperature-dependent small angle X-ray scattering (SAXS).A symmetric and an asymmetric PEO-I-P2VP were in the disordered state below ca.120 ℃ (but above the melting point of PEO), in which the two blocks mixed intimately with only thermal concentration fluctuations.A disorder-to-order transition (DOT) was observed upon heating and the phase transition was thermally reversible.In contrast to the system showing the conventional upper critical ordering transition, the interdomain distance of PEO-b-P2VP increased with increasing temnperature due to the greater stretching of the block chains to reduce the contacts or mixing of the PEO and P2VP monomers near the microdomain interface.The LCOT behavior was predicted by a theoretical calculation of the phase diagram of PEO/P2VP blends using a thermodynamic theory considering the disparity in solubility parameter and thermal expansion coefficient.