Van-der-Waals cluster formation of the hydrogen isotopologues in the windowless gaseous tritium source (WGTS) of the Karlsruhe Tritium Neutrino Experiment (KATRIN) influences the measured electron energy spectrum. Therefore it represents a systematic uncertainty for the neutrino mass measurement with KATRIN. In the cryogenic distillation column of the fusion fuel cycle, this cluster formation also limits the accuracy of the measurement of the concentration of liquid hydrogen isotopologues based on infrared (IR) absorption spectroscopy. This thesis presents the investigation of this cluster formation via IR absorption spectroscopy. In this thesis, a description of the band structure of the IR absorption spectra of the Van-der-Waals clusters of inactive hydrogen isotopologues is developed and the major contributions to the line shapes are determined. Based on this, the temperature dependency of the absorbance of the Van-der-Waals clusters is experimentally determined in the liquid and gaseous phase and conclusions towards the cluster formation are drawn.