We introduce a new approach for stabilizing oil-in-water nanoemulsions using a semisolid interphase formed by the phase separation of amphiphilic block copolymers from the organic phase. This system is illustrated using an amphiphilic diblock copolymer, poly(ethylene oxide)-block-poly(epsilon-caprolactone) (PEO-b-PCL), with commonly used oils. PEO-b-PCL can be miscible with oil at elevated temperatures (70-80 degrees C); however, polymer/oil demixing occurs as the temperature drops below the melting temperature of PEO-b-PCL (similar to 55 degrees C). A homogeneous polymer/oil mixture was dispersed in water at 80 degrees C to generate embryonic emulsions, and then the emulsion size was reduced to a nanometer range through microfluidic homogenization. The structure of the generated nanoemulsions is irreversibly frozen as they are cooled down to ambient temperature. The nanoemulsions stabilized by PEO-b-PCL show the excellent colloidal stability against thermal and chemical stresses, exhibiting no significant changes in the size distribution during incubation for 4 months at ambient temperature or 10 days at 60 degrees C. This study demonstrates that PEO-b-PCL is an attractive emulsifying material for practical nanoemulsion formulations requiring structural stability under a broad range of conditions.