Abstract Miscibility, crystallization and morphology of unsaturated polyester (UP) matrices, nanostructured with a poly[(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide)] (PEO-b-PPO-b-PEO) block copolymer (BCP) from 0 to 50 wt% has been investigated. Additionally, the role of each block on miscibility and morphology of cured mixtures was studied. Behaviours of non-reactive mixtures of UP thermosetting precursor with two BCPs composed of similar and strong immiscible central PPO block were compared. It was found that one BCP had PEO blocks with not enough molecular weight to compatibilize the PPO block with the UP thermosetting precursor at room temperature. Transmitted light intensity study of mixtures indicated that during curing at 35 °C no macrophase separation took place, contrary to the systems cured at temperatures equal or higher than 60 °C. Curing mixtures at 35 °C produced nanostructured matrices with almost unchanged transparency. Phase separation and miscibility of BCP with UP matrix were measured by means of DSC and DMA. AFM analysis showed worm-like morphology with diameters from 10 to 20 nm and length that evolved from 50 nm to 1 μm with increase of BCP content.