Ultralong alkanes crystallize into lamellae with a thickness that is an integer fraction of the extended chain length. On heating, they unfold or thicken to approach the equilibrium, extended chain form. Using high-resolution in-situ wide-angle X-ray scattering, carried out at the European Synchrotron Radiation Facility (ESRF), Grenoble, it has been shown that this thickening is accompanied by a contraction of the lattice, a contraction accompanying each thickening event between each integer folded crystal thickness. Prior to thickening, there is an increase in disorder, as measured by the change in peak width. In extended chain alkanes with the same crystal thickness the lattice simply expands thermally before melting.