Cavity ringdown spectra of the (A) over tilde-(X) over tilde electronic transition of all eight isomers of the pentyl peroxy radical are reported. Using the corresponding assignments from previously studied smaller alkyl peroxy radicals, assignments of origin bands are made for the pentyl peroxy isomers including some conformer-specific assignments for bands of a given isomer. Ab initio calculations also were performed to aid in the spectral assignments for neopentyl, t-butyl, and t-pentyl peroxies. In addition to the origins, vibrational bands have also been assigned for some species. Using the analyzed spectra, the relative reactivity of the primary, secondary, and tertiary hydrogen atoms in isopentane could be determined semiquantitatively.