Asphaltene precipitation and deposition is a serious problem in oil recovery, oil transportation and refinery operations. Understanding of the phase behaviour of systems containing asphaltenes is very important to develop effective asphaltene deposition mitigation strategies. In order to get a better understanding of asphaltenes and in particular to get more insight in relations between molecular structural features and observed macroscopic behaviour, the phase behaviour of two poly-aromatic molecules (violanthrone-79 and hexa-tert-butylhexa-peri-hexabenzocoronene) was studied. Although both compounds do not represent all specific features of real asphaltenes, it is expected that to some extend they do. For instance, real asphaltenes are ill-defined mixtures and hard to characterize, while the selected poly-aromatic compounds form a well-defined system with toluene and CO2. The poly-aromatic compounds were selected based on their molecular features and their tendency to self-assemble. The phase behaviour of solutions of these compounds in toluene was studied in the presence of different concentrations of carbon dioxide with a so-called Cailletet apparatus. The results in this work showed that the concentration of carbon dioxide affects the precipitation of the poly-aromatic compounds. At low concentration there is no precipitation, i.e., carbon dioxide acts as a co-solvent, while at higher concentration, carbon dioxide acts as an anti-solvent and the poly-aromatic molecule in the ternary system precipitates. As far as the phase behaviour is concerned, hexa-tert-butylhexa-peri-hexabenzocoronene resembles that of a real asphaltene.