Asphaltene aggregation and deposition are common concerns in reservoirs, and during production, transport, and refining of petroleum fluids. Polymer addition provides one approach for the modification and control of asphaltene behavior. Asphaltene nanoaggregates are stable in toluene, but when a nonadsorbing polymer like polystyrene is added, two liquid phases, one enriched in asphaltenes and toluene, and one enriched in toluene and polymer, are formed due to depletion flocculation. This phase separation mechanism impacts nanoaggregate but not molecular behaviors. In this work, an experimental phase diagram for Athabasca asphaltene + toluene + polystyrene mixtures including a two-phase critical point is presented, at 298 K. The compositions of phases and the critical point are compared with those arising in equivalent mixtures with Maya asphaltenes published previously. These experimental outcomes are also interpreted using a modified Fleer-Tuinier model, and the possibility of the development of an asphaltene separation process based on the depletion flocculation mechanism is discussed.