The ClC family encompasses two, at the first glance very different classes of proteins - anion channels and anion-proton exchangers. However, members of the transporter branch are able to function in a channel-like uncoupled mode with very high unitary transport rates. In addition, a newly solved structure of CmCLC, a eukaryotic ClC exchanger, suggests that transport might occur by a pore-mediated process. Therefore, it is not surprising that voltage-dependent gating of ClC channels is strongly dependent on the substrates transported by ClC transporters. This study is focused on the action of external and internal anions on fast gating of ClC channels. Based on data from the published literature we constructed a kinetic model in which a ClC channel opens only when a hypothetical site inside the permeation pathway is occupied by an anion. Making this site accessible from both side of the membrane in a voltage dependent manner is sufficient to explain the different effects of external and internal anions on the gating of ClC-0. For this isoform external anions enhance channel opening by shifting the voltage dependence of activation whereas internal anions increase the minimum open probability at negative potentials. In addition, this model has the ability to predict some of the experimentally observed variations among the different ClC channel isoforms - ClC-0, ClC-1 and ClC-2. Some similarities with the modulation of voltage dependent gating of mammalian ClC transporters by external anions suggests that the presented model might be applicable also for this class of proteins.