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A new simple and rapid catalytic kinetic method for the determination of trace amount of formaldehyde is described. The method is based on the catalytic effect of formaldehyde on the oxidation of Janus green by bromate in the present of sulfuric acid. The reaction monitored spectrophotometrically by measuring the decrease in absorbance of the reaction mixture at 618nm. The fixed-time method was used for the first 150s. For initiation of the reaction, under the optimum conditions, in the concentration range of 0.003–2.5μgml⁻¹ formaldehyde can be determined with a limit of detection 0.0015μgml⁻¹. The relative standard deviation of five replicate measurements is 2.3% for 1.0μgml⁻¹ of formaldehyde. The method was used for the determination of formaldehyde in real samples with satisfactory results.