In a recent paper of Miller et al. (Med. Phys. 43(5), 2141–2152 (2016), the voltage dependence polarity effect of microionization chambers was studied. It was identified that polarity effect arises from the difference in electric potential between the collecting electrode and the guard electrode, which in turn deforms the electric field and affects the charge collection. Nevertheless, the cause of such potential difference has not been identified. In this letter, we explain that the electric potential perturbation arises from the work function difference of the disparate materials electrodes (collecting vs guard electrodes in the particular case).