This method is a spectrophotometric determination of the carbonyl number, present as ketone or aldehyde carbonyl, and defined as “milligrams of carbonyl function per liter of sample using acetophenone as the standard.” The color intensity of the reaction product is determined both by concentration and the type of R-group presenting the carbonyl function. This method applies to aqueous solutions, and hydrocarbons and alcohols in the C5 to C17 range. The typical analysis range for carbonyl number is 1 to 100 mg/L carbonyl. Results of this analysis are primarily used as an indication of the oxygen exposure of UOP Parex® and Molex® feedstocks.