The thesis opens with an introduction in which the phenomenon of "knock" in internal combustion engines is reviewed. The use of metallic anti-knock compounds is discussed and the previous work on chromium compounds for this purpose is described There is some disagreement in the literature regarding the efficacy of chromium compounds as anti-knock agents. Chromium hexacarbonyl appears to he the most suitable chromium compound for testing as a fuel additive. The various methods of preparing the hexacarbonyl are reviewed. The remainder of the thesis is divided into five sections and two appendices. Section 1 describes attempts to prepare chromium hexacarbonyl by three different methods, various modifications of the methods being tried in order to improve the yields. Chromium hexacarbonyl is prepared by the method of Hieber & Romberg, the maximum yield being 9.5% (Claimed by Hieber & Romberg : 14%). The method involves the absorption of carbon monoxide by a violently agitated suspension of anhydrous chromic chloride in presence of a Grignard reagent.