A recently developed technique, i.e., two-dimensional infrared (2D IR) correlation spectroscopy, has been used to investigate the thermal-oxidative degradation of two networks based on a tetrafunctional epoxy resin. The use of the 2D IR approach to analyze time-resolved transmission spectra collected in situ during the high-temperature degradation process effectively enhanced the spectral resolution and revealed details about the reaction mechanism which remain undetected in the one-dimensional, frequency spectra. In particular, the sites of initiation of the auto-oxidative sequence were identified for the case of the plain epoxy resin, as well as the main competitive pathways through which the degradation reaction proceeds. With respect to the network modified by a thermosetting bis(maleimide) comonomer, which displays an IPN-like molecular structure, it was found that the epoxy component undergoes thermal oxidation via the same mechanism as for the plain epoxy network. A limited degradation was detected also for the bis(maleimide) component, whose stability represents the underlying reason for the enhanced temperature performances of the ternary system.