For demanding ultra-low-temperature applications such as environmental test chambers, refrigeration systems must operate reliably at temperatures of −80 °C and below. The selection of a suitable refrigerant is therefore not only constrained by its thermodynamic performance and environmental properties, but also critical regarding its behavior close to the solidification limit. While the freezing point is an important benchmark, the actual freezing process - including the behavior during subcooling, the crystallization dynamics, and the partial solidification - can significantly influence the operational stability of the system and so the applicability of a refrigerant below its nominal freezing temperature. In this work, the freezing behavior of several newly designed zeotropic refrigerant mixtures is investigated and compared with the commercially available refrigerant R-473A. The mixtures examined consist of R-744 (carbon dioxide), R-1132a (trifluoromethyl ethylene), R-125 (pentafluoroethane), R-116 (hexafluoroethane), R-23 (trifluoromethane), and R-32 (difluoromethane) in various combination and compositions. The aim is to identify refrigerant mixture that achieve freezing points below −80 °C, keeping in mind a low Global Warming Potential (GWP) for this field of application. The experimental analyses are supported by thermodynamic modelling. The experimental analysis focused on determining the beginning of freezing, the solidification range, and the potential of sub-freezing operation. The results show that some of the investigated zeotropic mixtures exhibit partial solidification without complete phase separation. Compared to R-473A, selected mixtures achieved comparable or lower freezing points down to −94 °C.