The mechanism of trapping and de-trapping of hydrogen atoms in tungsten is not yet fully understood. One method to investigate the underlying mechanism is to perform hydrogen isotope exchange in which trapped hydrogen atoms of one specific isotope (e.g. Deuterium) are replaced by those of a different isotope (e.g. Protium). Trapping and de-trapping of hydrogen atoms in tungsten can be described by two different models, the classical model and the fill-level model. Within those models the hydrogen isotope exchange proceeds differently. In contrast to the fill-level model the classical model should fail to explain hydrogen isotope exchange at low temperature where significant thermal de-trapping of hydrogen atoms is absent. The different hydrogen isotope exchange experiments at temperatures down to 150 K which were conducted to discriminate between the fill-level- and the classical model are presented and discussed. Furthermore, the experimental results are interpreted qualitatively in the context of a simple combinatorial model to highlight the fundamental exchange process, as well as quantitatively by applying the TESSIM-X diffusion-trapping code.