This investigation is concerned with the addition of promoters to molybdenum catalysts, and offers and explanation of the effect produced by the presence of a small quantity of a second substance in admixture with the catalyst. Autoclave Experiments —High pressure experiments were carried out in 3-litre rotating autoclaves heated by gas. For comparative purposes the initial cold pressure was 110 atmospheres; the duration of each run was 1 hour at the reaction temperature, which was controlled within ±2°. When cold, the pressure was reduced and the liquid products were examined, chiefly by distillation and in many cases by chemical analysis.