The thermal degradation of short-time heated carbon fiber (CF)-reinforced polyetheretherketone (PEEK, CF/PEEK) and high-density polyethylene (HDPE), in the range of 10 ms up to 3 min, was investigated by various detection methods such as differential scanning calorimetry, gravimetric analysis, and Fourier transform infrared spectroscopy (FTIR). Therefore, the polymers were heated by extrinsic and intrinsic heating methods, including induction heating, infrared heating, and laser heating, which provides the highest heat flux. The short-time heated polymers revealed thermal degradation after several milliseconds of laser irradiation as well as after several seconds of induction heating. CF/PEEK exhibited a decrease in crystallinity and melting extrapolated onset temperature with an increase in irradiation time and power. The FTIR results indicated that random chain scissoring started at the phenyl ring bonding, the ether bonding, and the aromatic hydrogen bonding, whether some new nonaromatic hydrogen bonding occurred. HDPE showed a decrease in melting peak temperature and some changes in the methylene stretching bonding.