Three nearly symmetrical ternary systems made of ternary blends of polystyrene, polyisoprene, polystyrene-block-polyisoprene copolymer have been investigated by small angle X-ray scattering (SAXS). The molecular weights of the homopolymers were varied, keeping the diblock length constant, to obtain ratios of homopolymer to diblock length, alpha, of 0.19, 0.38 and 0.74. For all experimental phase diagrams determined, a phase transition from lamellar to bicontinuous microemulsion, and then macroscopic phase separation have been observed with increasing homopolymer content. In addition, for alpha = 0.38 and 0.74, a region of coexistence of bicontinuous microemulsions and lamellae, between the periodically ordered phase (lamellar) and the channel of microemulsion is reported at low temperature, which transform into a pure bicontinuous microemulsions at a higher one. For alpha = 0.74, the concentration at which the macroscopic phase separation occurs is lower than the one obtained with alpha = 0.19 and 0.34, suggesting that, as the molecular weight of the two homopolymers is increased, the microemulsion morphology becomes less stable. The thermal fluctuations begin to destroy the microemulsion morphology in favour of macroscopic phase separated system. This observation is further confirmed by the study of the variation of the d-spacing for the lamellar phase, where no wetting of the diblock monolayers was noticed for alpha = 0.74 as compared to alpha = 0.19 and 0.38. (C) 2003 Elsevier Ltd. All rights reserved.