The reaction of hydrogen atoms (H) with isoprene (C5H8) in solid para-hydrogen (p-H2) matrices at 3.2 K has been studied using infrared (IR) spectroscopy. Mixtures of C5H8 and Cl2 were co-deposited in p-H2 at 3.2 K, followed by irradiation with ultraviolet light at 365 nm to produce Cl atoms from the Cl2, and subsequent irradiation with IR light to produce H atoms from the reaction of the Cl atoms with p-H2. The H atoms then react with the C5H8 to form H atom addition radicals (C5H9•). Upon 365-nm/IR photolysis, a multitude of new lines appeared in the IR spectrum and, based on the secondary photolysis behavior, it was determined that the majority of the new lines belong to two distinct chemical species, denoted as set X (an intense line at 776.0 cm−1 and 12 other weaker lines) and set Y (an intense line at 766.7 cm−1 and 12 other weaker lines). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level to determine the relative energetics and predict the IR spectra for the four possible isomers of C5H9• that can be produced from the addition of the H atom to the four distinct carbon atoms in C5H8. The newly observed lines of set X and Y are assigned to the 1,2-dimethylallyl (addition to carbon 4) and the 1,1-dimethylallyl (addition to carbon 1) radicals according to comparison with the predicted IR spectra of the possible products. The 1,2-dimethylallyl radical and the 1,1-dimethylallyl radical were predicted to be the most stable isomers, with the latter ∼8 kJ mol−1 lower in energy than the former and to have significantly lower barriers than the addition pathways for the two central carbons. The ratio of the 1,1-dimethylallyl to the 1,2-dimethylallyl radicals is estimated to be (1.0 ± 0.5):1.0, indicating that the two radicals are produced in similar amounts, which is consistent with the theoretical predictions that the barrier heights are very similar for the H atom addition to the two terminal carbon atoms.