In this work, a micromechanics-based framework for determining high-temperature thermophysical and mechanical properties of CFRPs undergoing thermal decomposition has been further developed. The goal is to develop a framework that enables to investigate the effects of the mass loss and material phase changes (i.e. formation of the secondary char and gas phases), heating rate, decomposition reaction, and pyrolysis pore pressure on the overall material properties. The present work is mainly focused on the investigation of the effects of the heating rate and decomposition reaction. Temperature- and heating rate-dependent volume fractions of the decomposing matrix constituent phases (polymer, char, and pores) are determined from the first-order Arrhenius kinetics. Representative volume elements (RVEs) with microstructures corresponding to specific temperature and heating rates are created and two-step numerical homogenization of RVEs has been performed to determine overall material properties of the AS4/3501-6 composite in a temperature range up to 900 K. It was determined that overall material properties exhibit strong dependence not only on the temperature but also on the heating rate. An increase in the heating rate shifts initiation of thermal decomposition to the higher temperature. As a result, composite properties are retained at the higher temperatures.