The crystal-to-hexagonal columnar mesophase transition in solution-grown single crystals of poly(di-n-propyl siloxane), PDPS, was studied in situ with selected-area electron diffraction and variable-temperature atomic force microscopy. It was observed that, upon the transition, the hexagonal columnar mesophase displays two mutually orthogonal orientations of the lattice. The 2a* and 2b* reciprocal lattice vectors of the alpha-phase crystal coincide with the a* vectors of each of the two differently oriented mesophase lattices. The in situ atomic force microscopy study of the evolution of a single PDPS crystal upon heating shows that crystal thickening can occur via a short-term dwelling in the mesophase. The high chain mobility in the mesophase can account for the exceptional range of accessible crystal thicknesses of PDPS, which is much larger than that observed for polyethylene single crystals.