The behavior of various force fields for pyridazine and 3,6-dichloropyridazine coming from different origins (i.e.. HF/6-31G*, MP2/6-311G**, BLYP/6-31G*, B3LYP/6-31G* previously scaled) and empirical force fields transferred from the benzene molecule is investigated. These potentials are subjected to a linear least-squares refinement with the available experimental information considered as observed data. Moreover, the transferability properties of the force fields for these related molecules are analyzed. From the refined force fields which gave the most satisfactory results in the fitting procedure, the physicochemical meaning of the force constants in terms of redundant simple internal coordinates in the unambiguous canonical form is studied. Although the results are not completely decisive for the pyridazine series, significant conclusions are drawn pertaining to their force fields, some of which are calculated for the first time. First, the utility of canonical force fields is shown through the force fields tranferred from benzene, which produce results for some symmetry blocks that are comparable to those obtained from quantum mechanical force fields. Second, we report the refined force fields in simple valence internal coordinates and observe that they give values for the ring-stretching force constants that agree with the electronic structure of this molecule, for which the aromatic character is shifted toward one of the Kekule structures that has double bonds between the C and N atoms and in the CC meta bond. All of this is in accordance with the low aromatic character of the pyridazine ring, especially as compared with the other azines. In addition, the calculated values for the torsion and CH and CCl bending force constants also are explained in the electronic structure of these systems.