Electron spin echo envelope modulation spectroscopy (ESEEM) was used to study the nuclear quadrupole interactions of the remote nitrogen of pyrazole and pyridazine in Cu(II)(dien)-heterocycle complexes. The quadrupole parameters were obtained by field-dependent ESEEM spectral simulations for the amino nitrogen of pyrazole (e2qQ = 2.86 MHz and eta = 0.74) and for the imino nitrogen of pyridazine (e2qQ = 5.35 MHz and eta = 0.02). The values obtained for pyridazine are comparable to those for metal-free solid pyridazine (Schempp, E.; Bray, P. J. J. Chem. Phys. 1967, 46, 1186). Since the quadrupole parameters for the amino nitrogen in metal-free solid pyrazole have not been reported, a comparison is made with the values obtained in the gas-phase measurement using microwave spectroscopy (e2qQ = 3.02 MHz and eta = 0.52 : Nygaard, L.; et al. J. Mol. Struct. 1974, 22, 401). Although the difference in quadrupole parameters for pyrazole in the Cu(II)(dien) complex as compared to values for pyrazole in the gas phase is small, a hydrogen-bonding effect can be attributed to the change observed. An explanation for the differences in quadrupole parameters between the amino nitrogen of imidazole and of pyrazole complexed to Cu(II)(dien) as a consequence of cross-substitution of nitrogen and carbon in the five-membered heterocycle is given.