A simple method of measuring charge in surface states as a function of the surfacepotential in MOS transistor or MOS capacitors is proposed. A constant d.c. current is fed into the gate of an MOS transistor and with the help of an operational amplifier, the gate voltage Vg with respect to the bulk is plotted as a function of gate charge Qg. The gate charge as a function of the surfacepotential φs can be directly read off from the Vg-Qg curve. As the silicon charge Qs(φs) is known from the literature the surface state charge can be easily determined as a function of φs. The method is illustrated with measurements on n-type and p-type MOS transistors. Finally the accuracy of the charge measuring method is discussed and a comparison with other interface state charge measuring methods is made from which the charge measuring method evolves as a method, attractive for its simplicity.