A method for determination of formaldehyde in air using 4-amino-3-hydrazino-5-sulfanyl-1,2,4-triazole as chromogenic reagent has been described. The blue coloration formed on an indicator strip was evaluated using reflectance spectrophotometry. The detection limit is at least 0.07 mg m⁻³ of HCHO (the WHO limit is 0.1 mg m⁻³) after taking an air sample for 1 min at the flow rate 1 dm³ min⁻¹.