This paper describes a sensitive and selective semi-continuous analyzer (FORMAL-FLU) for the fluorimetric determination of atmospheric formaldehyde. The method is based on the reaction between formaldehyde and Fluoral-P, producing the fluorescent derivative 3,5-diacetyl-1,4-dihydrolutidine (DDL) which, when excited at 410 nm, fluoresces at 510 nm. This analyzer consists of a gas diffusion chamber with a central microporous Teflon tube, with high gas permeability, filled with Fluoral-P solution. The air sample is pumped continually along the external tube and formaldehyde permeates the internal tube membrane, where it reacts selectively with Fluoral-P to form DDL. The analytical parameters of air sampling flow rate, sampling time interval and Fluoral-P reagent flow rate were optimized by the response surface method, using the Box-Behnken design. Under optimal conditions, air samples were pumped through the annular space of a gas diffusion chamber equipped with a central tube of microporous Teflon (17 cm long, 1.4 mm i.d., 2.15 mm e.d., 70% of porosity and 2 mu m average pore diameter) filled with Fluoral-P at 2.5 mL min(-1) for 30 min. After sampling, the mixture was directed at 1.3 mL min(-1) to a 2.0 m long reaction coil immerged in a thermostatic bath at 80 degrees C to improve the sample/reagent mixture and reaction rate, then to the fluorimetric detector (lambda(ex) = 410 nm lambda(em) = 510 nm), where the signal was acquired and recorded by an integrator. Peak heights were measured and these values were used in the calibration and determination steps. Under these conditions, the limit of detection was 0.55 ng mL(-1) and the coefficient of variation was 8.6%. The main advantage of the FORMAL-FLU system is its selectivity for formaldehyde, without significant interference from bisulfite and other aldehydes, especially acetaldehyde, low blank level, resulting in low detection limits and, above all, using a single sampling and measuring device equipment which allows for in situ measurements.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)