The phase behavior of a weakly interacting binary system composed of deuterated polystyrene (dPS) and poly(n-hexyl methacrylate) (PnHMA) was investigated by the turbidity measurement for the binary blend, and by small angle X-ray scattering (SAXS) and depolarized light scattering for the block copolymers. For the binary dPS/PnHMA blend, a new phase diagram involving both the upper critical solution transition (UCST) and lower critical solution transition (LCST) was observed by the delicate control of molecular weights between dPS and PnHMA. Whereas for the block copolymers such as dPS-block-PnHMA and PS-block-PnHMA, an order-to-disorder transition (ODT) on heating was observed within the experimental temperature range depending on the molecular weight. This coexistence of both a UCST and LCST in the dPS/PnHMA blend consequently represents the experimental evidence that the corresponding (d)PS-b-PnHMAs possess not only ODT, but also lower disorder-to-order transition (LDOT) character driven by a compressibility difference, although the latter is hindered by thermal degradation.