Molecular ionization potentials for series of compounds of the type X-C₆H₄-CN, X-C₆H₄CH₂-CN and X-C₆H₄-N(CH₃)₂ have been measured using the retarding potential difference technique (RPD. technique). The effect of the various substituents X is better correlated through the electrophilic Brown σp+ constants than through Hammett's σp values. No meta-para orientation effect is observed. For all the disubstituted phenyl compounds studied, the effect of the second substituent is affected by the electron-releasing power of the original substituent. Ionization potentials calculated by using the semi-empirical method of equivalent orbitals are in good agreement with the experimental values.