2-D woven carbon--carbon composites are a class of advanced structure composite materials which can maintain good mechanical property and high stiffness constants at elevated temperatures. The high failure rate during the initial charring process has been the major obstacle in attaining increased popularity of these materials. This paper reports the use of passive ultrasonics and mass spectroscopy techniques to monitor the charring process of 2-D woven carbon--carbon composites in order to accurately and sensitively detect in situ the development of macro-porosity. The passive ultrasonic technique with high-temperature waveguide sensors is demonstrated to effectively detect the acoustic emission (AE) associated with the microcracking events during the charring process. The mass spectrometry (MS) technique together with a unique ``sniffing'' tube is shown to detect major gas evolution, such as water, methane, and hydrogen during the initial pyrolysis. The gas information is important to understand the nature of catastrophic delamination. Post-process material characterizations, such as thickness shrinkage, weight loss, and microscopic characterization, were conducted to provide information about the physical changes due to the carbonization process and proved to correlate well with the AE and MS data.