Mixing barriers in the atmosphere play an important role in shaping the large-scale dynamical circulation, and greatly affect the transport of chemical species. While some success has been achieved in identifying the edge of the tropical pipe and the wintertime polar vortex barrier, finding the location of the tropopause mixing barrier has proven to be much more elusive. In this study, various methods are applied to identify the tropopause mixing barrier in the Canadian Middle Atmosphere Model, a three-dimensional coupled chemistry GCM. A GCM is used partly to provide a dynamically self-consistent data set, and partly because it is an important exercise in itself to assess the extent to which such models represent the tropopause mixing barrier. The methods studied include Nakamura's effective diffusivity, age of air, and PDFs of chemical species. In addition to quantifying the tropopause mixing barrier in the model, comparison of the different methods sheds light on their commonalities and differences.