The samples Co (0.0 6 x 6 0.6) were prepared using the standard ceramic method. The dielectric constant (ε 1−x Cu x Fe 2 O 4 0 ), dielectric loss (tan δ) and ac conductivity (σ ) were measured at room temperature as a function of the frequency. The results of the dielectric properties were explained on the basis of Maxwell–Wagner behaviour. The effect of Cu ion substitution on the ac electrical conductivity and dielectric properties at different frequencies, 0.1–1000 kHz, were studied. The data revealed that ε 0 and tan δ increased as the Cu ion increased, due to the increase in the number of vacancies at the iron site. The Cu ions change the conduction from an n- to a p-type. The Seebeck coefficient (dV/dT ) mVK gives a positive sign (p-type) for all samples up to ≈500 K, except for x = 0.6, for which it gives a negative sign. This indicates that the conduction is predominantly caused by electrons hopping between Fe 3+ ↔Fe 2+ . The mobility was calculated and found to increase with an increasing Cu content. 2+ −1 2+ 2+