Mulliken’s electroneg ativity measure, (I + A)/2, where I and A represent successive ionization potentials and electron affinity values, are correlated with atomic number Z in isoelectronic series. The isoelectronic series for which the correlations have been established are from 1s 1 to 1s 2 2s 2 2p 6 up to atomic number 20. A nonlinear variation is observed for all th e ten series. These variations can satisfactorily be explained by an equation in the form (I + A)/2 = a Z 2 + b Z + c . Finally, the coefficients a, b, and c are presented in tabulated form for all the ten isoelectronic series.