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The infrared spectrum of cyclopentane has been examined from 33 to $4000cm-1$. A series of pronounced Q-branches with a $5cm-1$ spacing was observed on top of the $CH2$ deformation. The Q-branch frequencies could be reproduced by assuming that the cyclopentane ring was undergoing pseudorotation. The spectroscopically derived parameter $mqo2$ was found to be $(11.0\pm 0.2)\times 10-40$ g. $cm2$.