Encapsulation of the cold junction of sheathed thermocouples by incorporating glass-to-metal seal, which substitutes for the conventionally used organic sealants, is expected to improve the reliability of thermocouples. In this study, the sealing of borosilicate glass with thermocouple wire materials, i.e., Ni-Cr and Ni-Al alloys, was investigated. Three types of specimens were prepared for both alloys: nonoxidized, mildly preoxidized, and deeply preoxidized. High-temperature sealing with borosilicate glass was conducted for each specimen. The results revealed that grain boundaries were preferentially oxidized in both alloys and that preoxidation significantly increased the bonding strength between the alloys and borosilicate glass. However, the preoxidation conditions corresponding to the optimal bonding strength differed between the two alloys, which necessitates tailored pretreatment of the metal surface. For the Ni-Cr alloy, the sealing interfaces indicated that the deeply preoxidized specimens achieved the optimal bonding strength because of the synergistic effect of the glass saturation mechanism and mechanical interlocking mechanism, whereas the mildly preoxidized specimens were affected by unexpected chemical reactions and the consequent microscopic defects at the sealing interface. For the Ni-Al alloy, the deeply preoxidized specimens exhibited inferior bonding strengths compared with the mildly preoxidized specimens, which was attributed to their overmature intergranular oxidation, in which the grain boundaries were eroded by molten glass during sealing and trapped the glass after cooling. The glass in the grain boundaries was stressed by the cooling contraction of the metals and became prone to cracking. This study reveals the microscopic bonding mechanisms for the sealing of glass to Ni-based alloys and the causes of failure, providing a new perspective on the regulation and optimization of the sealing interface.