The morphological structure of a fully cross-linked vinylester Atlac 580 resin has been studied by using modulated differential scanning calorimetry (MDSC). The results show that the networks exhibit a diphasic structure, characterised by the presence of two glass transition temperatures (T-g) It is believed that the two phases are composed of a polystyrene-rich phase and a vinylester-rich phase, which results from phase segregation during radical copolymerisation. An annealing process below the glass transition temperatures can make the separated phases readily visible. Furthermore, the results demonstrate that MDSC can offer better resolution and sensitivity than conventional DSC and dynamic mechanical thermal analysis. This enables overlapping glass transitions (with T(g)s closer than 10 degrees C) to be distinguished successfully.