Our research group has recently defined two new aromaticity indexes based on the analysis of electrondelocalization in aromatic species using the quantum theory of atoms-in-molecules. One of these indexes isthe para-delocalization index (PDI) that measures the electronic delocalization between para-related carbonatoms in six-membered rings. In this paper, we show that this index can be partitioned into individual molecularorbital contributions. We have applied this PDI decomposition to several polycyclic aromatic hydrocarbonsshowing that this partitioning provides new insight into the origin of aromaticity.