Spent caustic is a waste from oil refineries which is difficult to treat and dispose of due to its noxious properties such as high content in paraffins and asphaltenes hydrocarbons which form all sorts of emulsions and solutions, from 5 to 18% of free NaOH, up to 35 g of Cl-/L, phenol compounds, and organic and inorganic sulfur compounds. Its composition depends on the source of the fuel being desulfurated. This mixture is very reactive and therefore its characterization is difficult and dependent on the way the sample is handled. Samples have to be collected cold from the storage container with no more than 6 h of having been rejected from the process. The sampling container must be sealed and pressure-proof. Part of the sample has to be acidified in situ for certain analysis. The spent caustic from Tula refinery in Mexico has 320 g of COD/L from which 114 come from sulfur organic compounds (3.6 g COD/g of S), 153 from phenolic compounds (phenol 78 g of COD/L) and the rest from other petroleum derivatives such as asphaltenes, paraffins, and mercaptans. Present treatment processes involve incineration of spent caustics, and an effort is needed for biological treatment in order to cut maintenance costs.