The selective and sensitive gas chromatographic determination of trace amounts of styrene was studied. Styrene was converted into styrene dibromide by reaction with bromine, and the latter was detected with an electron-capture detector. The minimum detectable amount of the dibromide was about 0.01 ng and the sensitivity to the dibromide was about 500 times higher than the sensitivity of a flame-ionization detector to the styrene monomer. Dodecene-I and o- and m-cresol gave the greatest interference, but the cresols could easily be removed by reaction with a small amount of a dilute aqueous alkaline solution. The application to a practical specimen, namely urban air, is described.