Thermally breakable functional groups based on diketogulonic acid and salt formation between amine and carboxy groups were grafted onto the chains of unsaturated polyester (UP) molecules. These functional groups are stable at room temperature but can be broken at elevated temperatures. They can also react with isocyanates. The modified UP resin showed a fast viscosity rise during thickening and a stable viscosity during room temperature storage. Upon heating the thickened resin, we found that the viscosity was reduced to much lower than the viscosity of the regular resins. With a combination of modified and unmodified resins, the viscosity changes of the UP compounds can be controlled during moulding.