The rapid increase in the use of polymer composite materials recently has led to the investigation of their behavior under thermal loads. During thermal loading, for example a fire incident, the polymer materials start to degrade. The degradation rate is heating-rate dependent. In this work, medium heating rates were applied to a neat resin and two (2) resin batches doped with fire-retardant nanomaterials. The thermal decomposition behavior of these systems was investigated using multistage kinetics modeling, the kinetic triplets for each system were extracted using the van Krevelen methodology and finally the thermogravimetry tests were reconstructed. This is the first part of the work dealing with the fire response of polymer composites. It is a material characterization process and the outcome is used in Part B, where a progressive degradation material model is developed and implemented in a Finite Element code.