We report on small-angle neutron scattering experiments from diblock copolymers A−B dissolved in a mixture of partially compatible solvents a and b (an extended abstract of this work has been published in Bull. Am. Phys. Soc.2004, March). Block A is polystyrene, while block B is polyisoprene, polybutadiene, or hydrogenated polyisoprene. Solvent a is dimethylformamide, which is a good solvent for block A and a bad solvent for block B, and vice versa for solvent b, namely, cyclohexane. There is a range of temperatures and concentrations somewhat above as well as below the coexistence curve of the two solvents where the samples, though remaining macroscopically homogeneous, are microphase-segregated, presumably between a-rich and b-rich domains separated by interfaces covered with the diblock copolymer. For the various copolymers studied, we find cubic or hexagonal structures with local order only at high temperatures, but long-range ordered structures below the coexistence curve. The characteristic structural dimensions are in the range 60−110 nm and depend on both the polymer concentration and relative amount of solvents. We also show that the chains at the interface are slightly extended, as described by the value a = 0.65 of the Flory exponent that exceeds the value for free chains with excluded volume, a = 0.60.