Spin alignment of heteroatomic pi -conjugated systems, 2,6-, 2,4-, and 3,5-pyridinebis(phenylmethylene) (3,5-PY), in which heterocycle plays a ferromagnetic or anti ferromagnetic linker between carbenic units, has been studied by random orientation ESR spectroscopy. The electronic structures of the isomers are important for the understanding of spin alignment vs. topology of the heteroatomic pi -electron networks. In 2,6-PY and 2,4-PY, the perturbing nitrogen atoms are in active positions, giving rise to a larger influence on their spin structure than in 3,5-PY. A simple application of molecular Hund's rule is invalidated.