THE recent preparation of the borane B9Cl8H (ref. 1) prompts us to report some of our mass spectral work on the products arising from the thermal decomposition of diboron tetrachloride. When diboron tetrachloride decomposes spontaneously at temperatures more than 0° the main product is boron trichloride with lesser amounts of yellow B4Cl4 (ref. 2), red B8Cl8 (ref. 3) and dark red B12Cl11 (ref. 4) ; a dark purple solid has also been detected but in amounts too small to allow accurate classical analysis4.