Adsorption by carbon is a commonly used treatment method for the removal of organic contaminants from waste water. The surface properties of activated carbon can be modified to enhance their adsorption affinity for target contaminants. In this study, the influence of cerium oxide loading onto carbon materials with different pore sizes in improving their adsorption capacity for methyl orange, which is an anionic organic pollutant, was analyzed. The adsorption capacity was increased by seven times after cerium oxide loading in mesoporous carbon materials at a pH of 5, whereas in microporous materials there was no notable increase in adsorption capacity. Also, the increase in adsorption capacity was directly proportional to the weight percentage of cerium oxide loading in the mesoporous activated carbon material. Methyl orange adsorbs on cerium oxide through electrostatic interactions. More specifically, the anionic methyl orange is attracted to the localized positive charges on the cerium oxide supported on the carbon materials. Decreasing pH resulted in an increased adsorption capacity likely because of the abundance of protons in the solution, causing an increase in localized positive charge density on the cerium oxide. Results from this study may be extended to more complex systems, such as the adsorption of naphthenic acids contained in oil sands process water.