The static S(q) and dynamic S(q,t) structure factor in disordered and ordered solutions of ultrahigh molecular mass (3.6 x 10(6) g/mol) symmetric poly(styrene)-block-poly(isoprene) (SI) diblock copolymer were measured by photon correlation spectroscopy for wave vectors q on both side of the maximum S(q*) as a function of concentration and temperature in two solvents. At ambient temperature, the SI concentration c(ODT) at the disordered-to-ordered transition decreases with increasing solvent selectivity from toluene (4.2 wt %) to decaline (2.7 wt %). In the disordered region, deviation from solvent impartiality enhances and slows down the short (q > q*) length order parameter fluctuations and hence leads to deviation of S(q > q*) from its theoretical mean-field form. From ultrasmall X-ray and light scattering, the S(q) over a broad q range shows hexagonally arranged cylinders for the ordered solutions with decaline, suggesting a biased solvent partition in the two microphases. At c(ODT), a two-phase regime which is visually observed in both solvents due to light diffraction is comprised of two separated regions with different S(q) and S(q,t) only at the vicinity of q*. For a rigorous interpretation of these effects, the solvent selectivity along with the renormalization of the neat block copolymer composition should be considered in the framework of the blob theory.