In the present work, a sensitive and selective fluorimetric method for formaldehyde determination in air samples is described. The method is based in the reaction between formaldehyde and Fluoral P producing 3,5-diacetyl-1,4-dihydrolutidine, which, when excited at 410 nm, emits fluorescence at 510 nm.The Fluoral P was prepared by the reaction of 0.3 ml of acetic acid, 0.2 ml of acetylacetone and 15.4 g of ammonium acetate. Then, the volume was completed to 100 ml with deionized water. The Fluoral P obtained, if stored under refrigeration in the dark, can be used, safely, for 60 days.The calibration curve obtained with concentrations of formaldehyde in the range of 12 to 192 ng ml−1 (n=9) was Intensity=1.11C+0.06 (R2=0.9920). In the quantification of formaldehyde, air samples were passed at 1 l min−1, during 120 min, through glass impingers containing 40 ml of Fluoral P, followed by direct fluorescence measuring, or through two SEP PAK silica cartridges, coated with Fluoral P. The cartridges were eluted with 10 ml of Fluoral P solution and quantified by spectrofluorimetry. Under these conditions, the detection limit (S/N=3) obtained was 2.0 ng ml−1.The new methodology was validated by comparison with a well-known HPLC method in which formaldehyde was collected into SEP PAK C18 cartridges coated with 2,4 dinitrophenylhydrazine. The application of the t95% test did not show significant differences between the HPLC and either fluorimetric methodologies.This method has been used in the determination of gas phase formaldehyde in both indoor and outdoor sites. For the indoor site, the measured concentrations were in the range of 9.0 to 67.7 μl l−1, while for the outdoor site they were in the range of 16.8 to 38.8 μl l−1. Further, due to the ease of handling in field studies, the SEP PAK cartridges coated with Fluoral P were used. The formaldehyde concentrations thus determined, in outdoor sites, were in the range of 2.09 to 25.1 μl l−1. The main advantage of this analytical procedure is its selectivity for formaldehyde, without interferences from bisulfite and other aldehydes, especially acetaldehyde, and low blank level, resulting in low detection limits. In addition, very little sample preparation is required.