Equilibria for the formation of molecular electron donor-acceptor (EDA) complexes of a series of phosphine oxides with [60]- and [70]fullerenes have been studied in CCl4 medium. An absorption band due to a charge transfer (CT) transition is observed in the visible region. The vertical ionization potentials (I-D(v)) of the phosphine oxides determined from the CT transition energies using Mulliken's equation correlate well with those calculated by the semiempirical PM3 method. The experimental I-D(v) values also yield good estimates of the electron affinities of [60]- and [70]fullerenes. Among the phosphine oxides studied, triphenyl phosphine oxide, trioctyl phosphine oxide, and tri-n-butyl phosphine oxide form stable EDA complexes with [60]- and [70]fullerenes while the complexes of bis-2-ethylhexyl phosphonate with [60]- and [70]fullerenes decay slowly into secondary products. The kinetics of these reactions have been studied by UV-vis absorption spectrophotometric methods, and the rate constants of the decay processes and formation constants of the EDA complexes have been determined at four different temperatures from which enthalpies and entropies of formation of the complexes have been estimated.