Use of the DFT method with the B3LYP functional offers an efficient way for determining geometries of phosphole structures. It also provides inversion barriers in good agreement with experiment. The molecular orbital MP2 method is better suited to calculate the relative stability of the considered isomers. Combination of geometric (Julg index) and magnetic (NICS) criteria leads to an interesting approach to analyze the pi-electron delocalization and/or aromaticity in conjugated heterocyclic systems and can be used for investigation of much larger oligomers. It thus allows us to confirm that planarizing the phosphorus atom in phosphole monomers leads to a greater conjugation over the butadienic pi-system and that substituents, characterized by a strongly pronounced pi-system, exhibit a large extent of conjugation with the pi-diene moiety of the heterocyclic system. In push-pull systems such as 2-BH2-5-NH2-1H-phosphole, the electron delocalization along the pi-diene system is even more pronounced than in phospholes with strongly pronounced pi-systems. The employed approach is also used to analyze the relationship between electron conjugation and the phosphorus inversion barrier. More relevant for chemical applications is the result that the computational study provides a method of fine-tuning of phosphole building blocks, which may enable us to obtain higher pi-conjugation along the polymer backbone.