An analysis of published work, together with new data, has clarified the effect of chemical composition on the strength of wood pulp fibres. Among fibres of low fibril angle, and in nondegrading pulping processes, strength (expressed as breaking stress) is directly proportional to α-cellulose content over a wide yield range. This implies that the cellulose fibrils are the sole tensile-load-bearing elements; hemicellulose and lignin only serve as a matrix that transfers the stress under shear from fibril to fibril. However, pulping to a yield corresponding to an α-cellulose content higher than 80% tends to reduce fibre strength apparently because of the elimination of this stress-equalizing matrix. In cellulose-degrading processes, fibre strength falls below this expectation to an extent dependent upon the degradation. Thus a tool is provided that permits the degradative effect of new pulping or bleaching processes on fibre strength to be assessed. The value of zero-span strength as an index of fibre strength is confirmed.