Large quantities of formaldehyde are produced both by combustion sources such as automobile exhaust and by photooxidation reactions in the atmosphere. It has been suggested that formaldehyde is likely to be a factor in eye irritation by Los Angeles type smog. Consequently, there is much interest in the accurate microdetermination of formaldehyde in the atmosphere. One reagent proposed for the analysis of formaldehyde is chromotropic acid (1,8-dihydroxynaphthalene-3,6-disulfonic acid). The report discusses the use of this substance for the purpose.