In the course of systematic study of the solid-state polymerization of the cyclic oligomers of formaldehyde (─CH2─O─)m, it was found that pentoxane (─CH2─O─)5undergoes polymerization in the solid state to form polyoxymethylene (─CH2─O─)nby X-irradiation. The present article contains (a) the molecular and crystal structure determination of pentoxane, (b) the determination of the relative crystal orientation of pentoxane and the resultant polyoxymethylene, and (c) some discussions on the role played by the crystal structure of pentoxane in the solid state polymerization comparing with the cases of trioxane and tetraoxane.