A method has been developed for the quantification of the formation of formaldehyde during the advanced oxidation treatment (AOT) of wastewater destined for reuse. This method uses solid-phase microextraction (SPME) with on-fiber derivatization followed by gas chromatography-mass spectrometry (GC-MS) analysis. Based on calculated method detection limits (MDL) and ambient background levels, the method reporting (MRL) limit for formaldehyde was set at 10 microg/L. Precision for formaldehyde using this technique resulted in 23% relative standard deviation (RSD), while the internal standard, acetone-d(6), was only 6%. This method was used to evaluate the formation of formaldehyde in bench scale UV-AOT experiments using natural organic matter (NOM) fortified reagent water and tertiary treated wastewater effluent. Results suggest that the formation of formaldehyde increases in both the reagent water and wastewater matrices with increasing UV exposure and hydrogen peroxide concentrations, with overall higher concentrations of formaldehyde in the wastewater samples. No appreciable amount of formaldehyde formation was observed when UV was applied in the absence of hydrogen peroxide in both matrices tested.