Solid solutions of (Mg1-xNix)Al2O4 (x=0-1) have been prepared using the solid-state reaction method and their dielectric properties were measured at microwave frequencies. Formation of single-phase spinel-structured solid solutions was confirmed by X-ray diffraction analysis throughout the entire compositional range, and the measured lattice parameters varied linearly from 8.0810 A at x=0 (MgAl2O4) to 8.0445 A at x=1 (NiAl2O4) leading to a linear decrease in the unit cell volume from 527.71 to 520.59 A3. The ionic polarizability obtained, however, increased with the increase of x to a maximum at x=0.25 and decreased thereafter. It shows only a small variance in the epsilon(r) with Ni substitution for Mg and a maximum epsilon(r) of 8.21 was achieved for (Mg0.75Ni0.25)Al2O4 in response to the highest ionic polarizability and relative density. However, the Q x f value can be significantly promoted up to a maximum value of 130 000 GHz (measured at 15.4 GHz) at x=0.25. In addition, substitution of Mg by Ni also helps to decline the tau(f) value from -65 ppm/degrees C at x=0 to -53.5 ppm/degrees C at x=0.25.