The construction of coated-film electrodes is described. It is shown that they are miniaturizable, inexpensive, simple sensors for various cations and anions. They are similar to the coated-wire electrodes, but instead of a metal wire an extremely thin conductive metal layer manufactured in thin-film technology is used. Potassium and sodium electrodes studied so far show excellent electrode properties, i. e. near Nernst response over a wide concentration range, sometimes better than commercial ion-selective electrodes. Two film electrodes are used to develop a low cost sensor for the determination of the activity of ions in a simple way by potentiometric difference measurement; this solid-state sensor can be applied to biomedical measurements, especially for blood analysis.