We have used Raman spectroscopy to measure the axial stress distribution along a fibre during a quasi-static single fibre pull-out test. The stress distribution at the debonding front during the progress of debonding gives the maximum interfacial shear strength tau(s) directly. In addition, the stress distribution along the fibre after debonding can be used to evaluate the interfacial normal stress and the frictional coefficient. For the plasma treated high modulus polyethylene (PE) fibres used here, tau(s) is found to be 28 MPa by this method, while the apparent mean interfacial shear strength tau(a) obtained from the regular single fibre pull-out test varies from 3 to 15 MPa with the fibre embedded length l(e). Stress distributions derived from the shear-lag theory fit the experimental data for fully bonded fibres well, giving values for the shear-lag constant K and the stress transfer length 1/beta [1]. According to the shear-lag theory, tau(s) = betal(e)tau(a)coth(betal(e)) If beta can be found for a given system from Raman spectroscopy, tau(s), can be evaluated from the pull-out lest using this equation. The regular pull-out tests, corrected for residual stress and interfacial friction, give the same tau(s) but not the same beta or pull-out load as the slower Raman test. The shear-lag constant K can be expressed as a function of the matrix shear modulus and geometric terms. One of these terms is the effective interfacial radius, r(e), the radius at which the strain in the matrix equals the average matrix strain. Raman measurements indicate that r(e) is small, only four times the fibre radius. This result is supported by polarizing optical microscopy. The model of Greszczuk [2], which assumes a uniform shear within an effective interaction thickness b(i), gives a similar result. We find that b(i) = 20 mum, about twice the fibre radius. Using the pull-out test data, as for other fibre composites, b(i) and r(e) predicted by shear-lag theories do not agree with the results of microscopy to this extent. In these cases tau(s) is much larger than the yield strength of the matrix and as neither treatment considers plastic deformation of the matrix agreement should not be expected.