Mercerized cellulose in concentrated sulfuric acid was found to give rise to a new ultraviolet absorption band assigned to 5-hydroxymethyl-2-furfural (HF). The time dependence of the HF absorption shows that the saccharification process of cellulose readily takes place in 97% sulfuric acid in spite of the very low water content. The dehydration reaction of the glucose unit appears to induce the decomposition of cellulose to HF with high efficiency.