Abstract We present detailed studies on spinel structured Ni_0.5Co _x Zn_0.5−x Fe₂O₄ (x = 0.1, 0.2 and 0.3), synthesized by sol–gel combustion method. In addition to the structural properties, we present optical dielectric and magnetic response of this system as a function of x. From x-ray diffraction analysis, a single-phase formation in cubic spinel structure was confirmed with an average crystallite size (D _avg) variation of 46–61 nm. X-ray diffractograms were analyzed to evaluate the lattice constant (a), x-ray density (ρ _x) and porosity (P) in percentage. Energy dispersive x-ray spectroscopy was used to delimit the compositional mass ratios. Terahertz (THz) time-domain spectroscopy measurements were performed to investigate the refractive index ( n ˆ s ) of the samples at room temperature in a frequency range from 0.2 to 2.5 THz. Real and imaginary components of complex optical dielectric constant ( ε ˆ s ) with the increase of Co²⁺ content were deduced using THz spectroscopy data. The complex optical dielectric constant for the substitution of Co²⁺ content at x = 0.1, 0.2 and 0.3, was determined within 5% error ∼9.6 + i0.07, ∼10.9 + i0.2 and ∼11.7 + i0.5, respectively. The magnetic measurements were performed at a temperature range of 300–50 K using a superconducting quantum interference device vibrating sample magnetometer under an applied magnetic field of ±5 tesla. We found the highest saturation magnetization (M _s) of about 86.7 emu g⁻¹, 83.5 emu g⁻¹ and 80.4 emu g⁻¹ for the substitution of Co²⁺ content at x = 0.1, 0.2 and 0.3, respectively at room temperature.