The theology of Zuata heavy crude oil, saturated with carbon dioxide, was studied at a temperature of 50 degrees C and pressures up to 220 bar. Observations of phase behavior were also reported and used to interpret the rheological data. The crude oil is very viscous and non-Newtonian at ambient pressure, but when brought into equilibrium with CO2, the non-Newtonian behavior was weakened and eventually disappeared at high CO2 pressures. When diluted with 10 and 30 wt % toluene, the diluted crude oils and their mixtures with CO2 behaved as Newtonian fluids. The CO2-saturated mixture of the crude oil samples showed an exponential decrease in viscosity with increasing CO2 pressure but an increase in viscosity at higher pressures. During observation through a view cell, the CO2 dissolution caused a swelling effect on the original crude oil. When saturated with CO2, the swelling effect also occurred on the 10 wt % diluted crude oil but the volume of the oil-rich phase was decreased at higher pressures. However, for the 30 wt % diluted crude oil, a second liquid phase was observed on top of the oil-rich phase, at pressures higher than the CO, critical point. The mixture viscosity was inversely proportional to the CO2 solubility.