Standard potentials E(Fl./Fl-)(0) for 12 different 9-substituted fluorenide ions were measured in dimethyl sulphoxide (DMSO) by fast cyclic voltammetry. 9-Amino-fluorenide ions are found to exhibit two reversible single-electron waves in cyclic voltammetry showing. that the corresponding 9-aminofluorenyl radicals and fluorenium cations are stable. Standard potentials for the 9-aminofluorenide ion and for the 9-aminofluorenyl radical were measured. For 9-alkyl-, phenyl-, methoxy-, phenylthiofluorenide ions, only one single-electron irreversible wave was found and linear sweep voltammetric results indicate a DIM1 mechanism for less sterically hindered fluorenyl radicals. For severely sterically hindered fluorenyl radicals a more complicated mechanism was found to occur. The rate constants for the dimerization of the fluorenyl radicals were determined by comparison of experimental and simulated voltammograms, from the potential shift of oxidation potentials relative to standard potentials or by double potential step chronoamperometry.