Hydrothermal carbonization (HTC) is one possibility to concentrate energy of wet biomass in so-called hydrochar. To design technical HTC reactors in a rational way, the reaction kinetics of the four main components of biomass, i.e., carbohydrates, lignin, fats/oils, and proteins, should be known. To determine the kinetics of hydrochar functionalization, initially an appropriate analytical method must be developed. In the presented experiments, the hydrochar obtained by HTC of vanillin is investigated by diffuse reflectance infrared (IR) Fourier transform spectroscopy (DRIFTS). In order to assign the IR bands in the DRIFT spectra to different functional groups, temperature-programmed desorption (TPD) experiments are conducted, followed by a combination of DRIFTS and TPD. This TPD/DRIFT combination is applied as an analytical method to investigate the kinetics of hydrochar functionalization during HTC.