Despite the possible detrimental effect of oil on foam displacement processes in CO2 EOR the effect of oil on dense CO2 foams has received little attention relative to air foams or low density CO2 foams. Herein, the effect of both a first contact miscible hydrocarbon (dodecane) and crude oil on CO2/water (C/W) foams generated by a switchable surfactant, C-12-N-14(EO)(2) was examined at dense CO2 conditions at temperatures up to 120 degrees C (393 K) and 3400 psia (23 MPa). Upon increasing the fractional flow of dodecane, a gradual decrease in foam viscosity was observed as the foam becomes unstable. Since only two phases are present, traditional destabilization mechanisms for three phase oil/gas/water systems based on the entering and spreading are invalid. Therefore, an alternative mechanism is suggested whereby added dodecane strengthen the surfactant tail interactions with the nonaqueous phase (mixture of CO2 and dodecane) to shift the hydrophilic-CO2 philic balance (HCB) towards an unstable region. This mechanism is supported by a decrease of the CO2-water interfacial tension from similar to 5 mN/m to 0.5 mN/m for dodecane-water systems at 120 degrees C and 3400 psia. The effect of crude oil was more profound than for dodecane, whereby rapid destabilization of foam occurred at an oil fractional flow as low as 0.2. In this case, the immiscible portion of the crude oil can enter and spread at the lamellae to destabilize the foam as is evident in positive entering and spreading coefficients. Also, other foam destabilizing parameters such as temperature and capillary pressure were studied in the presence of oil and the results were consistent with those in the absence of oil.