Two field methods are proposed for determining 2 to 10 p.p.m. of nitrogen dioxide in air; a liquid absorption method based on a Griess-Ilosvay reaction in which nitrogen dioxide recovery is improved by the incorporation of sodium arsenite in an alkaline absorption solution; and a test-paper method which is unaffected by humidity and air, that involves the use of papers, treated with p-anisidine and glycerol, with a storage life of 3 months. In both methods the apparatus required is easy to manipulate and transport and less than 10 minutes are required for a complete determination.