This method is for determining diolefins in FCC derived C5 olefinic streams by gas chromatography (GC). The individual diolefins cannot be separated and individually identified, but are eluted and calculated as a composite. 3-Methyl-1,2-butadiene is not determined by this method. 1,2-Pentadiene and 2,3-pentadiene are not expected to be present in C5 streams and it is not known if they can be determined by this method. The lower determination limit is 10 mass-ppm.