Molecular complex formation between [60]- and [70]fullerenes with a series of phosphine oxides, namely, tri-n-octyl phosphine oxide, triphenyl phosphine oxide and tri-n-butyl phosphine oxide has been studied in CCl₄ medium by NMR spectrometric method. Both [60] - and [70] fullerenes have been shown to form 1:1 adducts with the above series of phosphine oxides. Formation constants (K) for all the complexes have been determined from the systematic variation of NMR chemical shifts of specific protons of the donors in presence of [60] - and [70] fullerenes. Trends in the values of K suggest that [70] fullerene binds stronger with the phosphine oxides relative to [60] fullerene.