In grass silage-based beef production systems, sugar cane molasses, of which the major carbohydrate is sucrose, is frequently an economically attractive supplement. However, the efficiency of utilisation of molasses appears to decline relative to barley, at high levels of dietary inclusion (Drennan, 1985) which may reflect differences in rumen fermentation. The objectives of this study were to (i) compare rumen fermentation in steers offered grass silage and a range of levels of a starch or sugar-based supplement, and (ii) since molasses has a low crude protein concentration, to determine the influence of the source of protein in a molasses-based supplement formulated to supply the same amount of nitrogen as a barley/soyabean mixture.