Abstract The compound 4-amino-3-penten-2-one is introduced as a member of a selective family of reagents for aldehydes. The stability, selectivity and reactivity of the reagent are described and its utility in automatic determinations of formaldehyde is demonstrated. It is shown that the condensation reaction between 4-amino-3-penten-2-one (called Fluoral-P) and formaldehyde gives a product detectable by fluorescence. The flow injection system utilizing this reagent can be used to determine formaldehyde in the lower. nanomole range.