X-ray methods have been used to investigate the changes in structure with temperature for the supramolecular structure formed by a poly(methacrylate) with tapered side chains. The polymer is poly[2-[2-[2-[2-(methacryloyloxy)ethoxy] 3,4,5-tris[[p-(dodecyloxy)benzyl]oxy]ben zoate]. Oriented fibers drawn from the columnar hexagonal (phi(h)) Liquid crystalline phase at -60 degrees C adopt an ordered structure at room temperature, in which ordered cylindrical moieties are packed on a hexagonal lattice with a approximate to 60 Angstrom. The internal structure of the cylinder has an axial repeat of c = 5.03 Angstrom, which contains eight monomer units based on the density. Integral half-widths for the small-angle and wide-angle maxima yield lateral crystallite widths of 820 and less than or equal to 50 Angstrom respectively. These data show that there is a high degree of perfection in the packing of the cylinders, but there is Little or no correlation of the internal structure from one cylinder to the next. At the transition to the phi(h) phase at similar to 40 degrees OC there is a loss of order in the structure within the cylinder : all that remains are limited correlations due to stacking of the side chains along the axial direction. The separation between the stacked units increases with temperature, while the cylinder diameter decreases steadily from 60.4 Angstrom at 39.9 degrees C to similar to 54 Angstrom at 95 degrees C. These structural changes are correlated with striking changes in the dimension of the fibers with temperature. The dimensions show little change with temperature in the ordered solid state, but after passing through the transition to the phi(h) phase there is a similar to 28% increase in length and similar to 14% decrease in width on increasing the temperature from 40 to 95 degrees C, just below the phi(h)-to-isotropic transition, pointing to a progressive rearrangement of the supramolecular organization.