The magnetic susceptibility χ and the Verdet constant V obtained along the crystallographic axis [110] of the rhombic TbAlO3 crystal have been investigated experimentally and theoretically in the temperature range between 78 and 300 K. We find that the Verdet constant V is not a linear function based on the magnetic susceptibility measured along the [110] axis over the same temperature range. It is shown that the temperature dependence of the Verdet constant measured along the [110] axis is associated with the absence of a contribution to the Faraday rotation of the Van Vleck “mixing” of the electronic states of the rare-earth ion Tb3+ that takes place in the presence of an external magnetic field.