Turbidimetric titration, viscometry, and end group determination have been used for the study of changes in MWD during the heterogeneous hydrolytic degradation of wood cellulose modified by previous crosslinking at the amorphous regions by urea dimethylol. It is shown that this method of modification of fine cellulose structure makes it possible to obtain (at the optimum crosslinking density) a cellulose with a more narrow MWD than that obtained in the hydrolysis of native cellulose. These results are quite consistent with the valve model of the structure of native cellulose in which the amorphous regions play the role of valves.