The auto-cascade refrigeration cycle offers higher reliability and lower manufacturing costs compared to the cascade refrigeration cycle, making it particularly attractive for applications involving significant temperature differences between the heat source and sink. However, this advantage comes with increased complexity in its operation. This article aims to develop a media package in Modelica to simulate auto-cascade refrigeration cycles, with the goal of enhancing understanding of their operation and control. The method is demonstrated using an R23/R134a mixture, a commonly used refrigerant combination in such applications. The media package is compatible with the Modelica Standard Library and is constructed using curve fits based on the REFPROP dataset. Unlike typical refrigeration systems, the media package for auto-cascade components includes the independent mixture composition (Xi) as an additional variable in function calls. A combination of polynomial and Chebyshev polynomial curve-fits for the refrigerant properties has been shown to provide an optimal balance between accuracy and computational efficiency. The article presents example simulations performed to demonstrate use of the media package with component models from the Modelica.Fluid package. A lumped model for the phase separator is developed and simulated with the R23/R134a media to demonstrate ideal phase separation.