A method is described for determining the oxides of nitrogen present in tobacco smoke. Components of the vapour phase of smoke are trapped on charcoal. The trap is eluted with sodium hydroxide solution and the oxidation products of nitric oxide appear as nitrite and nitrate salts. An ion-exchange technique is used to release and purify the free acids by removing excess of alkali. They are then neutralised, made just alkaline and evaporated to dryness. The residual salts are dissolved in sulphuric acid, iron(II) sulphate is added, and the absorbance of the pink complex is measured. The equivalent concentration of oxides of nitrogen is determined from a calibration graph obtained with nitrate standards. The method overcomes some of the problems of those reported previously, in that it determines both major products of oxidation of nitric oxide in smoke, and avoids the use of nitric oxide gas to prepare standards. Results have been compared with those obtained from separate determinations of the amounts of nitrite and nitrate formed by oxidation of the nitric oxide. All results are computed and reported as nitric oxide. The method has a coefficient of variation of 5·2 per cent.