Alcohol ethoxylates (AEOs) that contain a wide distribution of oligomers pose a challenge for ultraviolet (UV) absorbance detection due to the fact that the AEOs absorb strongly only in the range of commercial UV detectors between 190 and 200 nm. Most mobile phase components, with the exceptions of water and carbon dioxide, also absorb in this region. Ethoxylated hexadecanol and octadecanol were derivatized with disilazane-chlorosilane mixtures for the formation of phenyl containing silylethers. Derivatized samples were analyzed by supercritical fluid chromatography (SFC) coupled with both electrospray ionization mass spectrometry and UV absorbance detection. An increase in the number of phenyl groups incorporated into the derivatives increased the number of oligomers observed by UV detection. An increase in the number of oligomers detected increased the calculated average molar ethoxylate values. The average molar oligomer values calculated by SFC-UV for these alcohols were consistent with the nominal reported values.