This article presents a sigma acceptor strength scale for methyltrioxorhenium(VII) (MTO), one of the most versatile and useful high oxidation state organometallics ever described. The spectrophotometric titration of MTO with a series of N-donor bases in CCl4 gives formation constants (K-f) and enthalpies for the adduct formation reactions. An excellent linearity of log Kf with respect to the Hammett sigma constants of the substituents on the ligands was observed. The resulting p constant is proposed to be a good indication of the Lewis acidity of MTO. The enthalpies of adduct formation of N-donors with MTO also fit the ECW model to predict the values of EA and CA parameters for MTO. The parameters can be used to predict an acidity scale for MTO. These parameters also allow the chemists to predict and correlate quantitatively the enthalpies of MTO(.)Lewis base interactions. Significant chemical insights result from the fit of the data to the ECW model.