Nucleophilicities N_R–B of molecules R–B (R = F, Cl, Br, I, CN, NC, CH₃, SiH₃, CF₃, H) are determined from the equilibrium dissociation energies D_e of 70 hydrogen-bonded complexes R–B⋯HX (X = F, Cl, Br, I, HCN. HCCH, HCP). The change in N_R–B relative to N_H–B of H–B allows a quantitative measure of the inductive effect I_R of each group R because only the group R affects the electron density associated with the axial non-bonding electron pair carried by the boron in R–B. An alternative definition of I_R, suggested by the strong correlation of the N_R–B values with the minimum value σ_min of the molecular electrostatic surface potential on the 0.001 e Bohr⁻³ iso-surface along the R–B axis leads to excellent agreement between the two definitions.