The results of recent ultrasonic studies are presented on high temperature composites with emphasis on the broad sub-class of Carbon fiber/Phenolic Matrix composites. These materials were studied with emphasis on the evolution of macroporosity during carbonizing heat treatments. The ultrasonic instrumentation consisted of an Acoustic Emission (AE) Monitor and a Scanning Acoustic Microscope (SAM). An important aspect was the design and construction of the high temperature waveguides used for the AE sensor. Next the deployment of these waveguides was demonstrated for the in-situ detection of microcracking during pyrolysis in the range of temperatures from 25°C to 750°C. These measurements were coupled with interrupted-run studies, in which small coupons were removed from the furnace at various temperatures and then examined with the SAM to study the progress of microcracking and decomposition. The AE results showed reasonable run-to-run repeatability and were found to correlate well with the SAM results. A sharp peak in AE activity was found in the range from about 450°C to 600°C. Total accumulative counts, total energy release and peak of AE activity were seen to shift in a systematic way with changes in the relative heat-up rate. Qualitative explanations of these features are presented in terms of complementary results of matrix shrinkage, weight loss and gas flow measurements.