3,5- and 4,6-Cholestadienes were prepared by the removal of two molecules of hydrogen bromide from pseudo-cholestene dibromide (4,5-dibromocholestane) and beta-cholestene dibromide (5,6-dibromocholestane) respectively. Alcoholic potassium was found to be ineffective for the complete removal of hydrogen bromide but quinoline was found to act satisfactorily for the conversion of the dibromocholestanes to the cholestadienes. The cholestadiene (3,5-cholestadiene) obtained from 4,5-dibromocholestane was found to be laevorotatory, which indicates that one of the double bonds is in the 5,6-position; the other double bond would be in conjugation in the 3,4-position. The cholestadiene (4,6-cholestadiene) obtained from 5,6-dibromocholestane was found to be dextrorotatory, which indicates that one of the double bonds is in the 4,5-position; the other double bond would be in conjugation in the 6,7-position;3,5-Cholestadiene was purified by various methods until its melting point, specific optical rotation and refractive index were unaffected by further purification. Cholesterilene was prepared by five different methods and the samples of cholesterilene obtained were purified by various methods. The melting points, specific optical rotations and refractive indices observed were compared with those of 3,5-cholestadiene and the melting points and refractive indices of these products were found to be of the same value. The numerical agreement of the two highest laevorotations observed, namely those of the products obtained by the action of hydrochloric acid upon a mixture of allo- and epi allocholesterol and by the pyrolysis of cholesteryl methyl xanthogenate, leads to the indication that the products (including 3,5-cholestadiene) obtained by other methods still contain impurities which could not be removed by the procedure employed although the specific rotations observed are higher than previously reported. The treatment of cholesterilene and 3,5-cholestadiene with chromium trioxide yielded oxycholestenone (cholesten-4-dione-3,6) isolated as the monophenyl-hydrazone, which indicated their identity. (Abstract shortened by UMI.)