Two methods are proposed for determining the vapours of certain organomercury compounds in air, at concentrations in the region of 10 µg of mercury per cu.m. The mercurial vapours are collected either on a glass-fibre pad treated with cadmium sulphide, or on a fluidised bed of active carbon. Mercury vapour is released by heating, and is determined by comparing the colour produced on selenium sulphide test-papers with a range of standard colours. The cadmium sulphide method is applicable to the determination of ethylmercury chloride, ethylmercury phosphate, diphenylmercury and methylmercury dicyandiamide; the fluidised-bed method is also applicable to this range of compounds and, in addition, to diethyl mercury. Mercurial dusts can be determined by the cadmium sulphide method, and mercury vapour by a slight modification of the fluidised-bed technique. In both methods the apparatus used is simple to manipulate and the time needed for a complete determination is less than 30 minutes.