Procedures for measurement of interfacial tension as a function of pressure has been developed. The interfacial tension is deduced from the damping of thermally excited capillary waves, measured by interfacial laserlight scattering. The sample is contained in an optical high pressure cell and measurements can be performed at a maximum of 680 bar. We have performed measurements on microemulsion multiphase systems with low interfacial tension. We find that the interfacial tension on these systems can have a strong dependence on pressure.