The concentration and temperature dependence of the structure formed by poly(styrene-b-ethylene-altpropylene) (PS-PEP) diblock copolymer in 1-phenyldodecane, a nearly neutral solvent, was studied using small-angle X-ray scattering (SAXS). As the polymer concentration decreases from 100 wt% to 10 wt%, PS-PEP (M-n,M-PS = 42,000 g/mol, M-n,M-PEP = 62,000 g/mol, M-w/M-n = 1.04) solutions form lamellae, hexagonally packed cylinders, and disordered structure. The results show that the neutral solvent mainly attenuates the unfavorable interaction between PS and PEP blocks, and thus reduces the domain spacing. The slight selective nature of the solvent, however, leads to uneven solvent distribution on the two domains. In particular, SAXS analysis equipped with the form factor fitting results in quantitative estimate of averaged solvent composition in micro-phase separated domains. (C) 2016 Elsevier Ltd. All rights reserved.