In our material thermal aging studies, we need to accurately measure pressure of gas evolved from the material at elevated temperatures to reliably measure the quantity of total gas released. MKS 121A absolute capacitance manometer was selected for this in-situ pressure measurement because its electronic components are contained in a separated housing that can be located outside of the heating chamber. In this configuration, the sensor and the connecting cable can operate at an elevated temperature up to 150°C. To determine the performance of this type of gauges, pressure measurements were conducted with seven MKS 121A sensors of three different ranges (10, 100, and 1000 torr) in an oven at 25, 55, 80, and 145°C, and the measurements were compared with two reference pressure gauges (MKS 690A) located outside of the oven. The performance test results, the temperature effects on zero drift and pressure error, and the pressure measurement results of a static outgassing run with three materials at 145°C will be presented and discussed.