The decrease of the mechanical strength during bleaching is very moderate (Figure 1). In particular, the breaking length is reduced by 11%, indicating a rather soft conditions of pulp treatment at all stages of bleaching. This is confirmed by the fact that the parameters characterizing the degree of oxidative degradation of hardwood pulp (copper number - estimated content of carbonyl groups in cellulose and solubility in an alkaline solution of sodium zincate - the part of low molecular fractions of cellulose according to Kleinert) are slightly decreased until Pa-E delignification stage and insufficiently changed at further steps.