A method based on the use of an ion-exchange column, post-column derivatization with 2,4-pentane-dione (acetylacetone), and photometric detection for the determination of low levels of formaldehyde in aqueous solution is described. The lower limit of detection is 5 μg/L (ppb) and response is linear up to at least 10 mg/L. Reproducibility at the 1 mg/L level is 0.6% RSD. Determination of formaldehyde concentration in several aldehyde systems is demonstrated.