Oil in water emulsion is widely used in the petroleum, petrochemical, and many other industries, besides its presence as oil spills in seawater under atmospheric circumstances. Even so, the great number of publications on photodegradation research does not introduce obvious solutions for oil-emulsion photodegradation. The present work is an attempt made to remove emulsions of oil in water and seawater via the photocatalytic oxidation process. In this communication, synthetic wastewater samples were generated by emulsifying diesel oil in distilled water or in artificial seawater to investigate the effects of: (i) initial oil concentration, (ii) catalyst concentration, (iii) irradiation time, and (iv) pH. The photocatalytic activities of three commercial TiO2 types: Degussa P25, Hombikat UV-100, and Millennium PC50 were compared with two prepared TiO2 samples using an artificial UV source. The degradation rates of oil were found to be strongly influenced by all of the above parameters. The results showed that the Degussa P25 photocatalyst and the TiO2-SG prepared one were the most active among the photocatalysts under study, both in distilled water and in artificial seawater. The photocatalytic degradation of oil-emulsion was strongly hindered in artificial seawater. Removal of 92 and 43% of oil was achieved by Degussa P25 in distilled water and in artificial seawater, respectively, after 3 h irradiation. The optimized conditions obtained with an experimental design were 2 g/L TiO2 and pH 4.5.