In its native state, cellulose is characterized by a high degree of polymerization (DP) and, like most fibrous materials, good tensile properties. However, these characteristics are influenced by several chemical and physico-chemical processes. Some heterogenous degradation phenomena have been investigated in this study by evaluating the effect of light, temperature and acid hydrolysis on two cellulosic materials: flax and cotton. Both separate and combined effects were studied. The degradation reactions, which were evaluated by determining intrinsic viscosity values, develop in two steps at different rates. The possible relation between kinetics and yarn tensile properties were also investigated and are of importance in extrapolating useful indications on the future life time of the art object and on the use of consolidation which is an appropriate conservative intervention before lining becomes really necessary.