We have exposed pulp handsheets to two different degradative treatments, and compared their effects on strength properties. In accordance with earlier research, tensile stiffness and the shape of the stress-strain curve were independent of cellulose chain length and the fiber defects caused by the degradative treatments. When evaluated at the same viscosity, we found acid vapor treatment to be more detrimental to axial fiber strength than ageing treatment at elevated temperature and humidity. At the same mean fiber strength, acid vapor-treated handsheets show higher tensile strength. This is because acid vapor treatment is more heterogeneous than ageing treatment. The fiber network is able to compensate for the local defects, but not for the general degradation in fiber strength. In both treatments the mechanism for cellulose cleavage is assumed to be acid hydrolysis, the difference in the effect on fibers coming from the treatment conditions. Acid vapor treatment induces a fast reaction at defect sites and discontinuities in a fiber, while ageing treatment induces a slow, more homogeneous hydrolysis in fibers. Unlike axial strength, the Z-directional strength of softwood decreases only after harsh degradation when viscosity has dropped below 400 ml/g. The Z-directional strength of hardwood is already compromised at a viscosity of around 700 ml/g. The differences probably arise from differences in fiber ultrastructure.