Synthetic zeolites such as Silicalite have been shown to act as accelerators of fermentation of sugarcane molasses to ethanol by Saccharomyces cerevisiae. Silicalite added at 1 gl(-1) increased initial specific ethanol productivity in the fermentation of sugarcane molasses to ethanol at all concentrations of the substrate in the range of 100-300 gl(-1) total sugars. Increasing the additive level up to 30 gl(-1) did not enhance further the initial rates of conversion. However, with the accumulation of inhibitory levels of the product in the later phase of fermentation of high-gravity molasses, marked enhancement in the rate of conversion was observed at the higher level of 20 gl(-1) of Silicalite supplementation compared to the effect observed with 1 gl(-1). At the highest concentration of 300 gl(-1) of total molasses sugars, 130 gl(-1) product concentration was obtained in about 48 h, indicating about 95% conversion in the presence of 2% of the zeolite, while in the absence of the additive the product concentration leveled off at 120 gl(-1), attained in a period of about 72 h. Fermentation of molasses in semibatch mode at 150 gl(-1) total sugar concentration in the presence of 20 gl(-1) Silicalite resulted in uniformly enhanced conversion rates compared to fermentation in the absence of the additive. The periods required for completion of fermentation were about 16h in the presence of Silicalite and about 24 h in the absence of the zeolite in all the 10 cycles of use tried. The probable modes of action of Silicalite as an accelerator of the ethanolic fermentation of molasses through removal of inhibitory substances and changes in the flocculation behavior of the yeast are discussed.