A spectrophotometric method for the determination of formaldehyde in air is described, based on the colour reaction of formaldehyde, p-aminoazobenzene and sulphur dioxide in hydrochloric acid medium. Beer's law is obeyed at 505 nm in the range 2-12 mug of formaldehyde per 25ml of final solution (0.08-0.48 ppm). Optimum conditions for colour development, and possible interferences, have been studied.