A SHORT letter on an X-ray investigation of the structure of isoprene, chloroprene and butadiene at liquid air temperature was published in 19381, and a more detailed account of the work on butadiene has since been communicated to the Faraday Society2. It was pointed out that in the case of isoprene it was not found possible to get an X-ray diagram showing more than a broad liquid ring. In these experiments the isoprene did not appear to crystallize at all sharply, but solidified to a glassy mass. This may have been due to the impurity of the specimen. In the present case the isoprene was a very pure sample kindly prepared and purified for us by Dr. E. H. Farmer, of Imperial College.