The properties of corrosion product film formed on carbon steel pipelines under saltwater-oil-CO2 multiphase flow were examined by AC impedance methods. Experiments were made in a 101.6 cm ID 10 m long pipe. Experimental results showed that there are two reactions on the steel surface in a saltwater-oil mixture. One is charge transfer and the other is diffusion. By calculation of the charge transfer resistance, Rt, and Warburg impedance coefficient, σ, it is found that Rt in a saltwater-oil mixture is much higher than that in saltwater. In a saltwater-oil mixture, Rt and σ increase with immersion time. It is suggested that the porous corrosion film occurs on the surface of the steel and that the film becomes compact when the immersion time increases. The longer the exposure time, the more compact the film, resulting in a lower corrosion rate. Corrosion in carbon steel pipelines under multiphase flow has long been a problem in the oil industry.