The reversible redox process of electrochemically formed polyaniline is accompanied by an intercalation of anions and protons. The rate of ingress and egress of anions depends on the size of the anions, eg p-toluenesulfonate is much slower than anions like ClO4- or HSO4-. Because of the large current densities (especially during reduction of the film) a separation of these fast anions requires the use of microelectrodes. In this work small inorganic anions are compared, the mobility decreases in the sequence Cl- > HSO4- > ClO4- > NO3- > H2PO4- and the egress of the latter is 100 times slower than the egress of Cl-. The influence of size, electric field, and polarizability is discussed.