The electronic structure of isomeric dichloroiodobenzenes (C6H3ICl2) has been investigated by HeI/HeII photoelectron spectroscopy. The spectra were assigned by Green's functions calculations and comparison with the spectra of related dichlorobenzenes (C6H4Cl2). The careful analysis of '-orbital and halogen lone pair ionization energies, enabled us to analyze substituent effects in detail, describing them in terms of resonance, inductive and spin'orbit coupling interactions. The use of several descriptors simultaneously, rather than a single one, appears to be necessary to unravel the effects in polysubstituted benzenes. Our method of analysis is capable of describing substituent effect in detail even in polysubstituted benzenes where kinetic or equilibrium methods become impractical.