Abstract: A method of making a calibrated measurement of the contact potential difference between a probe electrode and the surface of a sample surface involves the use of a reference standard. The reference standard comprises an electrolyte solution in electrical contact with an electrochemical half-cell; the surface of the electrolyte constitutes the reference surface whose CPD can be measured. The method is intended for use with apparatuses such as a vibrating capacitor or Kelvin probe, and allows the absolute work function of a sample surface to be determined from measurements obtained in a gas atmosphere. An apparatus incorporating the reference standard, and an improved vibrating capacitor probe, are also disclosed.