The origin of gas-porosity in gold-based alloys produced via lost-wax casting in CaSO4-bonded investment has been identified using a combination of microanalytical and thermal techniques. The occurrence of gas-porosity is related to the thermal decomposition of CaSO4, which, with SiO2, constitutes the investment material and decomposes at a temperature very close to the casting temperature of some typical gold alloys used for jewelry production. The thermal reaction generates SO,, leading to gas-porosity and, therefore, to defective products. Furthermore, the results show the detrimental effect of thermal decomposition caused by the presence of ZnO, Cu2O, CuO, NiO, and Ag2O formed on the surface of the gold-based alloy during air melting or casting. Therefore, the solid-state thermal decomposition of CaSO4 in the presence of other ceramic oxides has been investigated and found to be related to their surface acid-base properties, measured as isoelectric points on the solid surface.