The acid-base and electrokinetic properties of some amino-polysiloxanes have been studied both experimentally, with potentiometric and electrophoretic techniques, and theoretically, in the framework of a surface ionization model based on the mass action law extended to cover the case where the excess free electrostatic energy of the surface plays the role of a charge-regulating factor. The applicability of potentiometric titration for surface charge determination has been discussed. It has been proven that in a traditional performance the potentiometric titration leads to incorrect results. An alternative, more sophisticated treatment of potentiometric data has been suggested which allows approximation of titration curves with good precision, calculation of isoelectric points (iep’s), and the collection of unique information, scarcely available by other means, on the composition of surface coatings in contact with an electrolytic solution. In particular, it has been seen that the surface of aminopolysiloxanes exists in a zwitterion-like form over a wide pH range around pH(iep).