In this paper, the behaviour of the optical band gap and ac conductivity of Mn-Zn nanoferrites with the variation of the Zn concentration is reported. It was observed that in spite of a slight increase in the optical band gap, ac conductivity increases with the increase in Zn concentration. The observed behaviour has been explained in terms of the dominating factor which plays a key role in determining its properties. Optical band gap has a strong dependence on the crystallite size while the ac conductivity depends on the availability of the charge carriers for conduction and applied frequency. With the variation in Zn concentration, the above-mentioned important factors change which affect both the properties. A comparative examination of the optical band gap and ac conductivity of the prepared samples clearly gives support to the hopping of the charge carriers as the conduction mechanism in ferrites.