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The phase behavior of deuterated polystyrene-block-poly(n-pentyl methacrylate) copolymers (dPS−PnPMA) was investigated by using small-angle X-ray (SAXS) and neutron (SANS) scatterings and rheology. This block copolymer exhibited a closed-loop type of phase behavior as did hydrogenated PS−PnPMA copolymers. The closed loop consists of two transitions:  lower disorder-to-order transition (LDOT) and upper order-to-disorder transition (UODT) occurring at a lower and higher temperature, respectively. The segmental interaction parameter (χ) between dPS and PnPMA blocks at various temperatures was obtained by fitting the incompressible random phase approximation theory to SANS results using a low molecular weight of dPS−PnPMA exhibiting a disordered homogeneous state over the entire temperature range. We found that at higher temperatures χ increased initially with increasing temperature …