Polystyrene/poly(vinyl methyl ether) (PS/PVME) blend films were prepared on hydrophilic SiO substrates by a dip-coating method from toluene solution. The phase-separation temperature of the (PS/PVME) blend films decreased with decreasing film thickness. The (PS/PVME) two-dimensional ultrathin film with a thickness comparable to the dimension of twice the radius of gyration of an unperturbed PVME chain did not show any distinct cloud point. Secondary ion mass spectroscopic (SIMS) measurement for the perdeuterated polystyrene/PVME (dPS/PVME) thin film revealed the selective adsorption of the dPS segments on the hydrophilic substrate in order to minimize the polymer-substrate interfacial free energy. X-ray photoelectron spectroscopic (XPS) measurement showed the enrichment of PVME at the air-polymer interface due to its lower magnitude of surface free energy compared with that of PS. However, the PVME weight fraction at the air-facing surface started to decrease with decreasing film thickness for thickness less than ca. 30 nm. Atomic force microscopic (AFM) observation revealed that the (PS/PVME) two-dimensional ultrathin film with a thickness of 25 nm was in an apparent phase-separated state forming droplet-like domains 200-500 nm in diameter and 20-40 nn in height. Scanning viscoelasticity microscopic (SVM) observation revealed that the droplet-like domains were composed of a PVME-rich phase. These results clearly indicated that the spinodal point decreased below room temperature with decreasing film thickness for thickness less than ca. 30 nm. The formation of the phase-separated domains can be explained by factors such as the negative spreading coefficient of PVME on the PS matrix and the large negative conformational entropy of a PVME chain against stretching.