Topological indices are determined for models of prospective pyridine extractants and the relationships structure-topological index and topological index-pyridine nitrogen basicity are described. The second order valence molecular connectivity index, the Balaban index and the Wiener number are appropriate structural descriptors of pyridine derivatives and, in several cases, permit the pyridine nitrogen basicity to be predicted. The relationships between pK(a) values and the structural descriptors are given and described.