P(S-b-MMA) and P(B-b-MMA) diblock copolymers (BCP) have been solubilized in a liquid epoxy. The obtained solutions have been characterized by rheology and small angle X-ray scattering (SAXS). As in the solid state, BCP can self-organize in solution to form well-ordered micellar structures. The two blocks respective roles have been clearly identified: at room temperature the PS or the PB block microsegregates while the PMMA block for which epoxy constitutes a good solvent, acts as a stabilizer of the microphase separation. The geometries and thermal stabilities of the ordered structures depend strongly on the molar masses and the chemical nature of the BCP blocks. For instance, the total molar mass of the BCP has to be high enough to obtain a periodic structure. On the contrary, if this molar mass is too high, too long relaxation times prevent the system from reaching its equilibrium. For the P(S-b-MMA) copolymer solution, a transition temperature from an order to a disorder state (T-ODT) is observed. The origin of this transition has been attributed to a solubilization of the PS domains around T-ODT. Macroscopically, this transition can be defined as a solid-like to a liquid-like transition. In the case of the P(B-b-MMA) copolymer solution, no order-disorder transition has been observed: it can be explained by the fact that the PB blocks are not soluble in epoxy at any temperature, up to T = 200 degrees C. (c) 2005 Elsevier Ltd. All rights reserved.