A flow injection procedure has been developed for the sensitive and selective determination of formaldehyde in reagents and beverages. The method involves on-line condensation of formaldehyde with 4-amino-3-penten-2-one, followed by fluorescence detection of the product 3,5-diacetyl-2,6-dihydrolutidine. Under optimized conditions, the relative standard deviation in peak height variation is 0.26% for formaldehyde (40.0 nmol ml–1), and the calibration graph is linear in the range 0.5–100.0 nmol ml–1. The detection is so specific for formaldehyde that the other compounds, including acetaldehyde, give no fluorescence response. Results obtained with the proposed method are in good agreement with those obtained by a standard spectrophotometric method. The flow injection procedure enables 12 samples to be analysed per hour on direct injection of the sample solutions, and it does not suffer from the high blank responses or interferences by contamination from the other aldehydes, which are found for previous methods. Its application to the analysis of reagents and beverages has revealed the widespread occurrence of formaldehyde as an impurity or a constituent.