A continuous flow method for the determination of formaldehyde in urban air has been developed based on the chemiluminescence reaction between hydrogen peroxide solution and gaseous formaldehyde in the presence of 3,4,5-trihydroxybenzonic acid (gallic acid) in alkaline solution. The emission produced by the chemiluminescence reaction is proportionate to the formaldehyde concentration from 10 ppb (10⁹) v/v to 50 ppm v/v. The limit of detection [signal-to-noise (S/N)= 3] for gaseous formaldehyde is 10 ppb. The relative standard deviation for six measurements of 25 ppb v/v of formaldehyde is 7.1%. Hydrocarbon, sulfur dioxide, carbon monoxide, carbon dioxide, nitric oxide, nitrogen dioxide, ozone, acrolein and benzaldehyde do not interfere; however, propinaldehyde and acetaldehyde interfere slightly. Generally, the concentrations of propinaldehyde and acetaldehyde in urban air are less than formaldehyde, therefore their interferences can be considered negligible.