The dielectric constant (∊ r) and the dielectric loss (tan σ) have been measured for samples of the type NiAl xFe 2-xO 4 ( x = 0, 0.2, 0.4, 0.6, 0.8 and 1). The measurements were carried out in the frequency range from 100 up to 10 5 Hz. The dielectric behaviour is found to be abnormal, giving a relaxation peak at a certain frequency. The peak shifts to lower frequency due to increasing aluminium substitution. The abnormal behaviour of the dielectric is explained due to two types of charge carriers. The temperature dependence of the dielectric relaxation frequency is measured and the activation energy of the dielectric relaxation is calculated.