A modification of the Trautz-Schorigin reaction into a flow-injection analysis configuration is described. Different approaches were used at the optimization of chemiluminescence determination of formaldehyde in water based on the reaction of formaldehyde, gallic acid and hydrogen peroxide in an alkaline solution. Detection system with a 218microl chemiluminescence cell was optimized by both a one-variable-at-a-time method, and a modified simplex method. A calibration graph is linear in the concentration range 4x10(-8) to 1x10(-5)M HCHO. The detection limit of formaldehyde for a signal-to-noise ratio of 3 is 4x10(-8)M. The relative standard deviations for 15 repeated measurements of 1x10(-6) and 5x10(-6)mol l(-1) HCHO are 4.32 and 3.33%, respectively. The analysis time is 1.5min. The method was applied to the determination of formaldehyde in urban rainwater. A comparison of results found by proposed method with those obtained by fluorimetric reference method provided a good agreement.