Abstract: In a weakly acidic medium of about 0.04 mol·L-1 H2SO4,the oxidation of methyl orange by potassium bromate was significantly catalyzed by formaldehyde. Linear relationship was kept between the magnitude of decrease in absorbance at 510 nm and the concentration of formaldehyde in the range of 0.08-0.48 mg·L-1 corresponding to the linear regression equation ΔA=1.563 2 C-0.044 1 (C: mg of HCHO per L) and with correlation coefficient of 0.998 5. Detection limit of the reaction was found to be 5.18×10-3mg·L-1. Kinetic behavior of the reaction was studied. Oxidation reaction of methyl orange was found to be of 0.5th order and catalytic reaction of formaldehyde to be of 1st order. Apparent activation energy of the reaction was determined and the value obtained was 74.53 kJ·mol-1. The proposed method was used in the determination of traces of formaldehyde in several water samples,giving values of RSD′s (n=8) less than 0.9%,and recoveries in the range of 95.1%-104.9%. Results obtained by this method were in consistency with the results found by the acetylacetone photometry.