A method of inhibiting the formation of fouling deposits occurring in spent caustic wash systems used for ethylene dichloride (EDC) manufacturing processes. Fouling occurs in these systems when they are in contact with EDC processing streams contaminated with oxygen-containing compounds, such as aldehydes. These deposits are formed in EDC processing streams contaminated with oxygen-containing compounds while in contact with a caustic solution having a pH>7. The invention comprises adding an effective deposit-inhibiting amount of a non-enolizable carbonyl compound to the caustic solution. Preferred non-enolizable carbonyl compounds are formaldehyde, glyoxal, benzaldehyde, p-anisaldehyde, formic acid, glyoxalic acid and paraformaldehyde.