An interfacial electric field intensity of a surface is measured by covalently binding a monolayer of a voltage sensitive chromophore to the surface and irradiating it with actinic radiation while it is in contact with a liquid and measuring a first fluorescence emission spectrum. A solution of the voltage sensitive chromophore dissolved in a sample of the liquid is also irradiated with actinic radiation and a second fluorescence emission spectrum is measured. The first and second fluorescence emission spectra are compared to determine the interfacial electric field intensity.