Quantification of gaseous products formed during the pyrolysis of ablative heat shield materials is essential for understanding the decomposition mechanism of these materials and can provide a foundation to develop improved materials response models. For this purpose, an experimental setup based on previous work has been assembled and calibrated, a data analysis method has been developed, and experimental campaigns have been conducted on three different materials relevant to atmospheric entry heat shields: pure SC-1008 phenolic resin, phenolic-impregnated carbon ablator domestic (PICA-D), and three-dimensional mid-density carbon phenolic insulation layer (3MDCP-IL). Product yields were derived from mass spectra of volatile pyrolysis products, collected as a function of sample temperature over the range of 25-1200 ºC at five linear temperature gradients with respect to time, in the range of ~1 – 25 ºC s^-1. Normalized molar and mass yields for each pyrolysis product and temperature gradient have been measured for each material, and thermogravimetric analysis curves have been synthesized from the temperature-dependent mass yield data and measurements of total mass loss. The data were used in conjunction with data in the literature to identify the temperature-dependent pyrolysis mechanisms for each material and identify the effects of temperature gradient, density, temperature nonuniformity, curing procedures, and carbon fibers.