Half-fringe photoelasticity was used to evaluate the two-dimensional state of stress in epoxy matrix micro-tensile specimens. The experimental technique is described in detail and demonstrated by the measurement of the stress distribution around a circular hole in a thin epoxy film subject to uniaxial tension. The results obtained using this photoelastic technique compare well with theoretical solutions. This system has enabled the mapping of stresses in the matrix of single (hemp) filament composites, strained parallel to the fibre axis, to be undertaken. Concentrations of stress were observed in the interphase region, adjacent to kink bands (micro-compressive defects) and stress concentration factors of approx. 1.4 were recorded. The role that these stress concentrations play in the failure of hemp fibre reinforced composites is discussed.