Dielectric properties of pyrochlores compositions from Bi2O3-Nb2O5-NiO system were analyzed. The dielectric properties are dominated with a low-temperature relaxation that is typical for Bi-pyrochlores. A vast pyrochlore homogeneity range that exists in this system allowed to correlate characteristics of the observed relaxations with a compositional variations within the A(2)O'- and B2O6 pyrochlore sublattice. It was possible to make a distinction between different influences of the two sublattices, which can be satisfactorily described by the existing relaxation model for Bi3/2ZnNb3/2O7. A new relaxor-like room temperature relaxation was found for Bi1.6Ni0.57Nb1.43O6.55.