The interference method for simultaneous measurement of the absolute refractive index and the thickness of plane parallel samples is developed. This method enables us to test solid (including optical glasses, crystals, plastics, and other sheet materials), liquid, and gaseous media over a wide spectral range. The experimental model for the realization of this method, the laser interference refractometer and thicknessmeter (IRT), is manufactured and investigated. The IRT accuracy of the refractive index measurement is not worse than that of the best modern goniometers and the accuracy of the thickness measurement corresponds to the interference accuracy. © 2000 Society of Photo-Optical Instrumentation Engineers.