Visual examinations of microscopic behavior of CO/sub 2/ flooding processes were conducted using a high pressure glass-bead packed transparent flow tube. The apparatus and techniques developed have made it possible to observe the physical phenomena of the displacement of oil by CO/sub 2/ under both miscible and immiscible conditions. Effects of CO/sub 2/ slug size and formation dip on oil recovery were also investigated. The results have been recorded in a series of magnified color photographs for after-run study and interpretation. Four natural crude oils ranging from API gravities of 15 to 48 were chosen for displacement tests. The majority of the tests were performed at a temperature of 120/sup 0/F and pressures from 1000 to 2500 psi. Three types of displacement, namely: (a) immiscible, (b) semi-miscible, and (c) miscible, were observed within the testing pressure range. Type (b) is a dispersion process in which oil is disintegrated into micro-size particles and transported in the CO/sub 2/ stream. More than one type of displacement could exist simultaneously in a flooding system. A CO/sub 2/ slug that is too large is wasteful and can cause early CO/sub 2/ breakthrough. A slug that is too small would allow the trailing water to channel through the oil bank and thus degenerate to an ordinary waterflooding; nevertheless, the oil recovery would be significantly increased over that achieved by a straight waterflooding.