Polarisation Fourier transform infra-red (FTIR) microspectroscopy was used to characterize the organisation and orientation of wood polymers in normal wood and tension wood from hybrid aspen (Populus tremula × Populus tremuloides). It is shown that both xylan and lignin in normal wood are highly oriented in the fibre wall. Their orientation is parallel with the cellulose microfibrils and hence in the direction of the fibre axis. In tension wood a similar orientation of lignin was found. However, in tension wood absorption peaks normally assigned to xylan exhibited a 90° change in the orientation dependence of the vibrations as compared with normal wood. The molecular origin of these vibrations are not known, but they are abundant enough to mask the orientation dependence of the xylan signal from the S₂ layer in tension wood and could possibly come from other pentose sugars present in, or associated with, the gelatinous layer of tension wood fibres.