The effects of chemical structure and molecular weight of three series of thermoplastic polyurethane-based (PU) low-profile additives (LPA) on the miscibility of styrene (ST)/unsaturated polyester (UP) resin/LPA ternary systems prior to reaction were investigated by using the Flory-Huggins theory and group contribution methods. The reaction kinetics during the cure at 110 degrees C and the cured sample morphology were also studied by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), respectively. The phase-separation characteristics of ST/UP/LPA systems during the cure, as revealed by the cured-sample morphology, and the DSC reaction-rate profile, could be generally predicted by the calculated upper critical solution temperature for the uncured ST/UP/LPA systems. Finally, based on the measurements for volume change and microvoid formation, volume shrinkage characteristics for the cured ST/UP/LPA systems have been explored.