An analysis has been made of time resolved wide angle X-ray scattering during the deformation of a 50%poly(ethylene terephthalate)/50%poly(ethylene 2,6naphthalate) copolymer at 120 degrees C at a draw rate of the order of 10 s(-1). During the initial fast deformation a meridional reflection associated with a smectic mesophase is observed to form. After the initial deformation, the mesophase reflection decays at a rate of about 1 s(-1). During the decay, the lateral halfwidth of the meridional reflection increases indicating attrition of the smectic mesophase regions from the lateral interfaces. The sample deforms in a non-uniform manner. After the applied deformation ceases, the sample continues to thin down in the region of the X-ray beam. This is attributed to the lack of crystallisation during the 5 s timescale of observation. Analysis of the azimuthal profile of the strong inter-chain diffuse diffraction at similar to 2.8 nm(-1) shows that the mesophase is associated with extended chains with a characteristic high alignment of segments along the deformation. During the period of decay of the mesophase it is proposed that the chain segments can be effectively divided into just two components: oriented mesophase and isotropic amorphous regions. The analysis indicates that the proportion of the mesophase varies from 80 to 40% during the observed decay period.