Pure henpentacontane (C51) and dopentacontane (C52) were synthesized. Solution-crystallized samples of both n-alkanes clearly showed solid-solid phase transition peaks on their DSC curves before melting, while melt-crystallized samples did not show these peaks. From X-ray and SEM observations, this phenomenon was ascribed to the fact that the high-temperature structures were frozen during crystallization from the melt. By solution crystallization at a temperature range in which the high-temperature phase exists in bulk crystals, single crystals consisting of the high temperature structures were obtained. In X-ray diffraction, the lattice parameters for C51 were determined from Weissenberg photographs, and that of P2(1)/a for C52, from oscillation observations. It was noted that the even-odd effect of the number of carbon atoms still works with the high-temperature structures.