Chem. or electrochem. oxidn. of pentaisopropylcyclopentadienylsodium yields yellow-green crystals of the pentaisopropylcyclopentadienyl radical (1), which is very air sensitive under formation of tetraisopropylcyclopentadienone (2). 1 Has been characterized by EPR/ENDOR, proton NMR spectroscopy, and mass spectrometry. X-ray structural anal. of 1 reveals stacks of eclipsing, equally spaced, planar five-membered rings. Cyclovoltammetric measurements as well as a low-energy first PES ionization band of 1 at 7.02 eV are in accordance with facile generation of the pentaisopropylcyclopentadienyl cation 5 from pentaisopropylcyclopentadienyl chloride (4) or bromide (3) and antimony pentachloride. In accordance with MNDO/CI calcns. cation 5 is a ground-state triplet. Its EPR spectrum at 115 K shows a zero field splitting parameter D = 0.1615(4) cm-1 with coincidence of the x and y lines, indicating no detectable distortions from axial asymmetry and only weak electron-delocalizing properties of the iso-Pr substituents.