A linear solvation energy relationship is employed to characterize the specific surface polarity of organically modified silica particles. The surface polarity of silicas can be quantitatively described by three independent terms, the dipolarity/polarizability (pi*), the hydrogen-bond donating ability (alpha), and the hydrogen-bond accepting (HBA) ability (beta). These terms can he defined using the Kamlet-Taft solvent parameters alpha, beta, and pi* as a reference system. The HBA property beta of organically modified silica surfaces and anions adsorbed to silica has been determined by measuring the energy of the d-d* transition (nu(max)) of adsorbed Cu(tmen)(acac)X-+(-) [X- = Cl-, Br-, NO3-, CH3CO2-, and B(C6H5)(4)(-)]. nu(max) correlates linearly with the beta parameter of the solvents. Cu(tmen)(acac)B+(C6H5)(4)(-) is used as a beta indicator for the organically functionalized silicas because the nonnucleophilic anion B(C6H5)(4)(-) does not interact with residual silanols. Cu(tmen)(acac)Cl-+(-), Br-, NO3-, and CH3CO2- are adsorbed to residual silanols due to the higher basicity of the counteranion. The correspondence between the residual silanol acidity and the basicity of the functional groups of specific silica samples is discussed.