A flow method for the determination of formaldehyde in air is presented. A cylindrical wet effluent diffusion denuder is used for continuous collection of formaldehyde from air into deionized water as denuder liquid. Collected formaldehyde in the denuder concentrate is on-line determined by fluorescence 2,4-pentandione method. The collection efficiency of HCHO is quantitative at the air flow rate of 0.5 l min-1. The calibration graph is linear in the range 2-300 microg m-3 HCHO. The limit of detection is 1.6 microg m-3 HCHO. The relative standard deviations for 210-6 M HCHO and 110-5 M HCHO are 2.57 % and 1.06 %, respectively. Acetaldehyde, acetone, formic acid, acetic acid and NO2 and O3 do not interfere. Interference of SO2 is suppressed. Analysis time is 2.5 min.