The effect of reaction temperature, reaction time, initial chemical oxygen demand (COD), excess O-2 (EO), and catalyst concentration on the efficiency of catalytic wet air oxidation (CWAO) treatment of refinery wastewaters consisting of various oily wastes, water, heavy metals, toxic compounds, etc. was studied. In general, COD decreases substantially with temperature, EO, and oxidation time. Addition of homogeneous Co2+-based catalysts reduces COD by as much as 98.7%.