Abstract The effect of steam on the buffer material used in high-level radioactive waste (HLW) repositories has led to incessant uncertainty on the safety assessment because of the likelihood of loss in isolating potential. Hydraulic properties of compacted bentonite during vapor treatment is not fully and directly understood because of the difficulty of performing necessary tests under repository conditions. This article presents a vapor testing device conceived of and developed to monitor the evolution of swelling pressure and gas pressure during steam treatment and hydraulic conductivity after treatment under constant volume conditions. Although heat lag and thermal expansion were observed, the heating procedures were cautiously controlled and the swelling pressures were corrected, accordingly. Furthermore, the hydraulic properties of compacted bentonite during and after treatment were tested with a water-to-solid ratio of 1.3 and at different temperatures. Overall, the developed apparatus has demonstrated itself as an effective tool to better comprehend the interaction between bentonite and water vapor as regards to the temperature gradient and boundary conditions.