Purpose:The purpose of this study was to determine the proper prerequisite conditions for gas injection and the steps for applying gas injection for obturator fabrication. Materials and Methods:Optimal prerequisite conditions were investigated by using thermocoupling to verify exothermic changes in the resin during polymerization. Three experimental groups were designated: in the control group (C), resin was packed into the mold without argon gas injection; in the gas injection group (GI), after resin packing, gas was injected into the resin bulb in the mold; and in the plunger/gas injection group (PGI), after resin packing, the syringe plunger was drawn out before gas injection. Results:The 55°C mold temperature and addition of 0.3% DMPT to the monomer liquid caused differences in exothermic temperature increase in various parts of the resin. Without gas injection and plunger drawing, porosity was observed in the thick bulbs of groups C and PGI. In group PGI, a balloon-like elliptic hollow was observed. Compared to group C, the weight reduction in group PGI was approximately 10%. Conclusion: Proper mold temperature with secondary venting and the addition of DMPT were required before the injection of gas. The resulting prosthesis could be fabricated in one step and required no resin seal. Int J Prosthodont 2004;17:345–349.