We present a preliminary work for a general method of computing the partition of σ and π electronic effects of a given atom A or substituent R on a given substrate. In this aim, the nuclear charge Z∗ of a fictitious hydrogen atom H ∗ is fitted in order that the A–H ∗ (or R–H ∗ ) bond be purely covalent, i.e. the Mulliken electron population be one electron on H ∗ . We obtain this way entities of the same electronegativity as A or R, thus having a comparable σ effect, without any π effect. The values of Z∗ obtained for A–H ∗ diatomic molecules (A=H–Br) exhibit a good linear correlation with the Allred–Rochow scale of electronegativity, as it could be expected on theoretical grounds. The method, applied to R–H ∗ molecules, allows a determination of the electronegativity of a variety of polyatomic R substituents, and provides H ∗ (R) having the same inductive effect as R. These results are discussed by comparison with some previous theoretical and experimental data. As an example of application, the partition of σ and π contributions of R on the 13 C chemical shifts in a series of monosubstituted benzenes RC 6 H 5 has been computed.