The influence of a Jahn-Teller ion on magnetization,g-values and linewidth in Znx Fe3−xO4 and Ni1−x Znx Fe2O4 spinel ferrites has been investigated by introducing small quantities of Cu2+ ions. The magnetization of Cu substitution in both the systems but the curie temperature is affected only in FeZn ferrites. The lattice constant in NiZn is significantly reduced while in FeZn there is only a slight increase. The change ingeff on copper substitution,Δgeff, is very different in the two systems. In NiZn,Δgeff has a uniform value, ∼ 0·2, for all values ofx while in FeZn it has a peak atx=0·3 and vanishes atx=0 andx⩾0·5. The ferromagnetic linewidthΔH does not change significantly on copper substitution in both the systems. An explanation based on the perturbation produced due to the local lattice distortion by the Jahn-Teller mechanism explains the observed results satisfactorily.