The cellulosic fraction of biomass feedstocks can be converted to ethanol, a promising alternative fuel, using the simultaneous saccharification and fermentation (SSF) process. This process integrates the enzymatic hydrolysis of cellulose to glucose with the fermentation of glucose to ethanol. Its performance depends on the characteristics of the biomass, the quality of the cellulase enzyme complex, and the behavior of the fermentative organism. This study of cellulose conversion progress in batch mode indicates that during the early stage of SSF, cell growth is the rate-determining step in ethanol production. At later times, however, and for most of the duration of the SSF process, enzymatic hydrolysis becomes the limiting factor. Further experimental probing has shown that cellulose accessibility to the enzyme is the key cause of the decreased rate of cellulose hydrolysis. It is therefore concluded that accessibility needs to be enhanced to improve the productivity of the SSF process.