A vinyl ester resin (VER) with negligible carboxyl and hydroxyl groups was prepared from bisphenol A epoxy resin and alpha-methacrylic acid in combination with styrene. Simultaneous interpenetrating polymer networks (SINs) consisting of this VER and polyurethane (PU) could be obtained with negligible inter-component chemical binding between the two networks. The synthetic courses of these SINs were monitored with differential scanning calorimetry. While the amount of PU formation remained almost the same, the copolymerization of VER was greatly affected by temperature, by the composition of SINs and by the molecular crosslinking nature of both components. Furthermore, the thermal behaviour during the synthesis of SINs from VER containing hydroxyl groups was examined and showed that, when the secondary hydroxyl groups in VER were included for the metering of isocyanate in the feed for PU formation, inter-component chemical binding between the two networks should occur, resulting in a completely different molecular architecture of SINs.