The aim of this work was to characterise the solute solute interactions which exist in the CO2 + n-dodecane + 3,7-dimethyl-1-octanol +1-decanol system. High pressure bubble- and dew-point data were therefore measured for four different mixtures between 308 K and 358 K and pressures up to 27.5 MPa, with total solute mass fractions ranging between 0.631-0.015. The data were measured using the static synthetic visual phase detection method and experiments were conducted using a high pressure variable volume view cell. The effect of the solute solute interactions was also analysed by modelling the phase behaviour of the mixtures with the RK-ASPEN model in Aspen Plus*. The measured and predicted data indicated that significant solute solute interactions exist in the quaternary system. The solute solute interactions were also found to result in complex phase behaviour, as mixtures consisting largely of 1-decanol exhibited temperature inversions and co-solvency was found in n-dodecane rich mixtures.