Infrared spectra of crystalline cyclopentane and cyclopentane-d10 have been measured at different temperatures between 4000 and 200 cm−1. Spectroscopic evidence shows that form I (− 120°C) and form II (− 140°C) have the same degree of disorder as the liquid. In the ordered crystal form III (− 180°C) the molecule has a rigid bent conformation. By the mixed-crystal technique the nature of the observed splittings has been established. This has permitted us to show that the site symmetry of the molecule in form III is C1, and packing concepts together with the spectroscopic data led to the choice of C2h5 as the possible space group of the lowest-temperature form.