A number of low-angle parameters have been evaluated for a high-modulus carbon fibre, the most useful is considered to be l_p, Porod's `distance of heterogeneity'. When I^−½ is linearly related to θ², l_p=2a, where a is the correlation length obtained from the (I^−½, θ²) plot. The parameter l_p and the internal surface area have been evaluated for carbon fibres heat treated in the range 1000-2800°c. There is good correlation between the low-angle parameters and strength, but a marked discontinuity at about 1900°c. This discontinuity is related to a change from highly cross-linked graphite with many small pores and small crystallite size, to graphite with little crosslinking, a lower internal surface, but larger individual pores and crystallites where crystallite perfection is an important factor. High strength in stress-graphitized carbon fibres would appear to be a result of strain-induced crystallization with the removal of lattice defects.